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@@ -1,450 +1,367 @@
-# Thread safety of the PSA subsystem
+# Thread-safety of the PSA subsystem
 
-Currently PSA Crypto API calls in Mbed TLS releases are not thread-safe. In Mbed TLS 3.6 we are planning to add a minimal support for thread-safety of the PSA Crypto API (see section [Strategy for 3.6](#strategy-for-36)).
+Currently, PSA Crypto API calls in Mbed TLS releases are not thread-safe.
 
-In the [Design analysis](#design-analysis) section we analyse design choices. This discussion is not constrained to what is planned for 3.6 and considers future developments. It also leaves some questions open and discusses options that have been (or probably will be) rejected.
+As of Mbed TLS 3.6, an MVP for making the [PSA Crypto key management API](https://arm-software.github.io/psa-api/crypto/1.1/api/keys/management.html) and [`psa_crypto_init`](https://arm-software.github.io/psa-api/crypto/1.1/api/library/library.html#c.psa_crypto_init) thread-safe has been implemented. Implementations which only ever call PSA functions from a single thread are not affected by this new feature.
 
-## Design analysis
+Summary of recent work:
 
-This section explores possible designs and does not reflect what is currently implemented.
+- Key Store:
+    - Slot states are described in the [Key slot states](#key-slot-states) section. They guarantee safe concurrent access to slot contents.
+    - Key slots are protected by a global mutex, as described in [Key store consistency and abstraction function](#key-store-consistency-and-abstraction-function).
+    - Key destruction strategy abiding by [Key destruction guarantees](#key-destruction-guarantees), with an implementation discussed in [Key destruction implementation](#key-destruction-implementation).
+- `global_data` variables in `psa_crypto.c` and `psa_crypto_slot_management.c` are now protected by mutexes, as described in the [Global data](#global-data) section.
+- The testing system has now been made thread-safe. Tests can now spin up multiple threads, see [Thread-safe testing](#thread-safe-testing) for details.
+- Some multithreaded testing of the key management API has been added, this is outlined in [Testing-and-analysis](#testing-and-analysis).
+- The solution uses the pre-existing `MBEDTLS_THREADING_C` threading abstraction.
+- The core makes no additional guarantees for drivers. See [Driver policy](#driver-policy) for details.
 
-### Requirements
+The other functions in the PSA Crypto API are planned to be made thread-safe in future, but currently we are not testing this.
 
-#### Backward compatibility requirement
+## Overview of the document
 
-Code that is currently working must keep working. There can be an exception for code that uses features that are advertised as experimental; for example, it would be annoying but ok to add extra requirements for drivers.
+* The [Guarantees](#guarantees) section describes the properties that are followed when PSA functions are invoked by multiple threads.
+* The [Usage guide](#usage-guide) section gives guidance on initializing, using and freeing PSA when using multiple threads.
+* The [Current strategy](#current-strategy) section describes how thread-safety of key management and `global_data` is achieved.
+* The [Testing and analysis](#testing-and-analysis) section discusses the state of our testing, as well as how this testing will be extended in future.
+* The [Future work](#future-work) section outlines our long-term goals for thread-safety; it also analyses how we might go about achieving these goals.
 
-(In this section, “currently” means Mbed TLS releases without proper concurrency management: 3.0.0, 3.1.0, and any other subsequent 3.x version.)
+## Definitions
 
-In particular, if you either protect all PSA calls with a mutex, or only ever call PSA functions from a single thread, your application currently works and must keep working. If your application currently builds and works with `MBEDTLS_PSA_CRYPTO_C` and `MBEDTLS_THREADING_C` enabled, it must keep building and working.
+*Concurrent calls*
 
-As a consequence, we must not add a new platform requirement beyond mutexes for the base case. It would be ok to add new platform requirements if they're only needed for PSA drivers, or if they're only performance improvements.
+The PSA specification defines concurrent calls as: "In some environments, an application can make calls to the Crypto API in separate threads. In such an environment, concurrent calls are two or more calls to the API whose execution can overlap in time." (See PSA documentation [here](https://arm-software.github.io/psa-api/crypto/1.1/overview/conventions.html#concurrent-calls).)
 
-Tempting platform requirements that we cannot add to the default `MBEDTLS_THREADING_C` include:
+*Thread-safety*
 
-* Releasing a mutex from a different thread than the one that acquired it. This isn't even guaranteed to work with pthreads.
-* New primitives such as semaphores or condition variables.
+In general, a system is thread-safe if any valid set of concurrent calls is handled as if the effect and return code of every call is equivalent to some sequential ordering. We implement a weaker notion of thread-safety, we only guarantee thread-safety in the circumstances described in the [PSA Concurrent calling conventions](#psa-concurrent-calling-conventions) section.
 
-#### Correctness out of the box
+## Guarantees
 
-If you build with `MBEDTLS_PSA_CRYPTO_C` and `MBEDTLS_THREADING_C`, the code must be functionally correct: no race conditions, deadlocks or livelocks.
+### Correctness out of the box
 
-The [PSA Crypto API specification](https://armmbed.github.io/mbed-crypto/html/overview/conventions.html#concurrent-calls) defines minimum expectations for concurrent calls. They must work as if they had been executed one at a time (excluding resource-management errors), except that the following cases have undefined behavior:
+Building with `MBEDTLS_PSA_CRYPTO_C` and `MBEDTLS_THREADING_C` gives code which is correct; there are no race-conditions, deadlocks or livelocks when concurrently calling any set of PSA key management functions once `psa_crypto_init` has been called (see the [Initialization](#initialization) section for details on how to correctly initialize the PSA subsystem when using multiple threads).
 
-* Destroying a key while it's in use.
-* Concurrent calls using the same operation object. (An operation object may not be used by more than one thread at a time. But it can move from one thread to another between calls.)
-* Overlap of an output buffer with an input or output of a concurrent call.
-* Modification of an input buffer during a call.
+We do not test or support calling other PSA API functions concurrently.
 
-Note that while the specification does not define the behavior in such cases, Mbed TLS can be used as a crypto service. It's acceptable if an application can mess itself up, but it is not acceptable if an application can mess up the crypto service. As a consequence, destroying a key while it's in use may violate the security property that all key material is erased as soon as `psa_destroy_key` returns, but it may not cause data corruption or read-after-free inside the key store.
+There is no busy-waiting in our implementation, every API call completes in a finite number of steps regardless of the locking policy of the underlying mutexes.
 
-#### No spinning
+When only considering key management functions: Mbed TLS 3.6 abides by the minimum expectation for concurrent calls set by the PSA specification (see [PSA Concurrent calling conventions](#psa-concurrent-calling-conventions)).
 
-The code must not spin on a potentially non-blocking task. For example, this is proscribed:
-```
-lock(m);
-while (!its_my_turn) {
-    unlock(m);
-    lock(m);
-}
-```
+#### PSA Concurrent calling conventions
 
-Rationale: this can cause battery drain, and can even be a livelock (spinning forever), e.g. if the thread that might unblock this one has a lower priority.
+These are the conventions which are planned to be added to the PSA 1.2 specification, Mbed TLS 3.6 abides by these when only considering [key management functions](https://arm-software.github.io/psa-api/crypto/1.1/api/keys/management.html):
 
-#### Driver requirements
+> The result of two or more concurrent calls must be consistent with the same set of calls being executed sequentially in some order, provided that the calls obey the following constraints:
+>
+> * There is no overlap between an output parameter of one call and an input or output parameter of another call. Overlap between input parameters is permitted.
+>
+> * A call to `psa_destroy_key()` must not overlap with a concurrent call to any of the following functions:
+>     - Any call where the same key identifier is a parameter to the call.
+>     - Any call in a multi-part operation, where the same key identifier was used as a parameter to a previous step in the multi-part operation.
+>
+> * Concurrent calls must not use the same operation object.
+>
+> If any of these constraints are violated, the behaviour is undefined.
+>
+> The consistency requirement does not apply to errors that arise from resource failures or limitations. For example, errors resulting from resource exhaustion can arise in concurrent execution that do not arise in sequential execution.
+>
+> As an example of this rule: suppose two calls are executed concurrently which both attempt to create a new key with the same key identifier that is not already in the key store. Then:
+> * If one call returns `PSA_ERROR_ALREADY_EXISTS`, then the other call must succeed.
+> * If one of the calls succeeds, then the other must fail: either with `PSA_ERROR_ALREADY_EXISTS` or some other error status.
+> * Both calls can fail with error codes that are not `PSA_ERROR_ALREADY_EXISTS`.
+>
+> If the application concurrently modifies an input parameter while a function call is in progress, the behaviour is undefined.
 
-At the time of writing, the driver interface specification does not consider multithreaded environments.
+### Backwards compatibility
 
-We need to define clear policies so that driver implementers know what to expect. Here are two possible policies at two ends of the spectrum; what is desirable is probably somewhere in between.
+Code which was working prior to Mbed TLS 3.6 will still work. Implementations which only ever call PSA functions from a single thread, or which protect all PSA calls using a mutex, are not affected by this new feature. If an application previously worked with a 3.X version, it will still work on version 3.6.
 
-* **Policy 1:** Driver entry points may be called concurrently from multiple threads, even if they're using the same key, and even including destroying a key while an operation is in progress on it.
-* **Policy 2:** At most one driver entry point is active at any given time.
+### Supported threading implementations
 
-Combining the two we arrive at **Policy 3**:
+Currently, the only threading library with support shipped in the code base is pthread (enabled by `MBEDTLS_THREADING_PTHREAD`). The only concurrency primitives we use are mutexes, see [Condition variables](#condition-variables) for discussion about implementing new primitives in future major releases.
 
-* By default, each driver only has at most one entry point active at any given time. In other words, each driver has its own exclusive lock.
-* Drivers have an optional `"thread_safe"` boolean property. If true, it allows concurrent calls to this driver.
-* Even with a thread-safe driver, the core never starts the destruction of a key while there are operations in progress on it, and never performs concurrent calls on the same multipart operation.
+Users can add support to any platform which has mutexes using the Mbed TLS platform abstraction layer (see `include/mbedtls/threading.h` for details).
 
-#### Long-term performance requirements
+We intend to ship support for other platforms including Windows in future releases.
 
-In the short term, correctness is the important thing. We can start with a global lock.
+### Key destruction guarantees
 
-In the medium to long term, performing a slow or blocking operation (for example, a driver call, or an RSA decryption) should not block other threads, even if they're calling the same driver or using the same key object.
+Much like all other API calls, `psa_destroy_key` does not block indefinitely, and when `psa_destroy_key` returns:
 
-We may want to go directly to a more sophisticated approach because when a system works with a global lock, it's typically hard to get rid of it to get more fine-grained concurrency.
+1. The key identifier does not exist. This is a functional requirement for persistent keys: any thread can immediately create a new key with the same identifier.
+2. The resources from the key have been freed. This allows threads to create similar keys immediately after destruction, regardless of resources.
 
-#### Key destruction short-term requirements
+When `psa_destroy_key` is called on a key that is in use, guarantee 2 may be violated. This is consistent with the PSA specification requirements, as destruction of a key in use is undefined.
 
-##### Summary of guarantees in the short term
+In future versions we aim to enforce stronger requirements for key destruction, see [Long term key destruction requirements](#long-term-key-destruction-requirements) for details.
 
-When `psa_destroy_key` returns:
+### Driver policy
 
-1. The key identifier doesn't exist. Rationale: this is a functional requirement for persistent keys: the caller can immediately create a new key with the same identifier.
-2. The resources from the key have been freed. Rationale: in a low-resource condition, this may be necessary for the caller to re-create a similar key, which should be possible.
-3. The call must not block indefinitely, and in particular cannot wait for an event that is triggered by application code such as calling an abort function. Rationale: this may not strictly be a functional requirement, but it is an expectation `psa_destroy_key` does not block forever due to another thread, which could potentially be another process on a multi-process system. In particular, it is only acceptable for `psa_destroy_key` to block, when waiting for another thread to complete a PSA Cryptography API call that it had already started.
+The core makes no additional guarantees for drivers. Driver entry points may be called concurrently from multiple threads. Threads can concurrently call entry points using the same key, there is also no protection from destroying a key which is in use.
 
-When `psa_destroy_key` is called on a key that is in use, guarantee 2. might be violated. (This is consistent with the requirement [“Correctness out of the box”](#correctness-out-of-the-box), as destroying a key while it's in use is undefined behavior.)
-
-#### Key destruction long-term requirements
-
-The [PSA Crypto API specification](https://armmbed.github.io/mbed-crypto/html/api/keys/management.html#key-destruction) mandates that implementations make a best effort to ensure that the key material cannot be recovered. In the long term, it would be good to guarantee that `psa_destroy_key` wipes all copies of the key material.
-
-##### Summary of guarantees in the long term
-
-When `psa_destroy_key` returns:
-
-1. The key identifier doesn't exist. Rationale: this is a functional requirement for persistent keys: the caller can immediately create a new key with the same identifier.
-2. The resources from the key have been freed. Rationale: in a low-resource condition, this may be necessary for the caller to re-create a similar key, which should be possible.
-3. The call must not block indefinitely, and in particular cannot wait for an event that is triggered by application code such as calling an abort function. Rationale: this may not strictly be a functional requirement, but it is an expectation `psa_destroy_key` does not block forever due to another thread, which could potentially be another process on a multi-process system. In particular, it is only acceptable for `psa_destroy_key` to block, when waiting for another thread to complete a PSA Cryptography API call that it had already started.
-4. No copy of the key material exists. Rationale: this is a security requirement. We do not have this requirement yet, but we need to document this as a security weakness, and we would like to satisfy this security requirement in the future.
-
-As opposed to the short term requirements, all the above guarantees hold even if `psa_destroy_key` is called on a key that is in use.
-
-### Resources to protect
-
-Analysis of the behavior of the PSA key store as of Mbed TLS 9202ba37b19d3ea25c8451fd8597fce69eaa6867.
-
-#### Global variables
-
-* `psa_crypto_slot_management::global_data.key_slots[i]`: see [“Key slots”](#key-slots).
-
-* `psa_crypto_slot_management::global_data.key_slots_initialized`:
-    * `psa_initialize_key_slots`: modification.
-    * `psa_wipe_all_key_slots`: modification.
-    * `psa_get_empty_key_slot`: read.
-    * `psa_get_and_lock_key_slot`: read.
-
-* `psa_crypto::global_data.rng`: depends on the RNG implementation. See [“Random generator”](#random-generator).
-    * `psa_generate_random`: query.
-    * `mbedtls_psa_crypto_configure_entropy_sources` (only if `MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG` is enabled): setup. Only called from `psa_crypto_init` via `mbedtls_psa_random_init`, or from test code.
-    * `mbedtls_psa_crypto_free`: deinit.
-    * `psa_crypto_init`: seed (via `mbedtls_psa_random_seed`); setup via `mbedtls_psa_crypto_configure_entropy_sources.
-
-* `psa_crypto::global_data.{initialized,rng_state}`: these are bit-fields and cannot be modified independently so they must be protected by the same mutex. The following functions access these fields:
-    * `mbedtls_psa_crypto_configure_entropy_sources` [`rng_state`] (only if `MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG` is enabled): read. Only called from `psa_crypto_init` via `mbedtls_psa_random_init`, or from test code.
-    * `mbedtls_psa_crypto_free`: modification.
-    * `psa_crypto_init`: modification.
-    * Many functions via `GUARD_MODULE_INITIALIZED`: read.
-
-#### Key slots
-
-##### Key slot array traversal
-
-“Occupied key slot” is determined by `psa_is_key_slot_occupied` based on `slot->attr.type`.
-
-The following functions traverse the key slot array:
-
-* `psa_get_and_lock_key_slot_in_memory`: reads `slot->attr.id`.
-* `psa_get_and_lock_key_slot_in_memory`: calls `psa_lock_key_slot` on one occupied slot.
-* `psa_get_empty_key_slot`: calls `psa_is_key_slot_occupied`.
-* `psa_get_empty_key_slot`: calls `psa_wipe_key_slot` and more modifications on one occupied slot with no active user.
-* `psa_get_empty_key_slot`: calls `psa_lock_key_slot` and more modification on one unoccupied slot.
-* `psa_wipe_all_key_slots`: writes to all slots.
-* `mbedtls_psa_get_stats`: reads from all slots.
-
-##### Key slot state
-
-The following functions modify a slot's usage state:
-
-* `psa_lock_key_slot`: writes to `slot->lock_count`.
-* `psa_unlock_key_slot`: writes to `slot->lock_count`.
-* `psa_wipe_key_slot`: writes to `slot->lock_count`.
-* `psa_destroy_key`: reads `slot->lock_count`, calls `psa_lock_key_slot`.
-* `psa_wipe_all_key_slots`: writes to all slots.
-* `psa_get_empty_key_slot`: writes to `slot->lock_count` and calls `psa_wipe_key_slot` and `psa_lock_key_slot` on one occupied slot with no active user; calls `psa_lock_key_slot` on one unoccupied slot.
-* `psa_close_key`: reads `slot->lock_count`; calls `psa_get_and_lock_key_slot_in_memory`, `psa_wipe_key_slot` and `psa_unlock_key_slot`.
-* `psa_purge_key`: reads `slot->lock_count`; calls `psa_get_and_lock_key_slot_in_memory`, `psa_wipe_key_slot` and `psa_unlock_key_slot`.
-
-**slot->attr access:**
-`psa_crypto_core.h`:
-* `psa_key_slot_set_flags` - writes to attr.flags
-* `psa_key_slot_set_bits_in_flags` - writes to attr.flags
-* `psa_key_slot_clear_bits` - writes to attr.flags
-* `psa_is_key_slot_occupied` - reads attr.type (but see “[Determining whether a key slot is occupied](#determining-whether-a-key-slot-is-occupied)”)
-* `psa_key_slot_get_flags` - reads attr.flags
-
-`psa_crypto_slot_management.c`:
-* `psa_get_and_lock_key_slot_in_memory` - reads attr.id
-* `psa_get_empty_key_slot` - reads attr.lifetime
-* `psa_load_persistent_key_into_slot` - passes attr pointer to psa_load_persistent_key
-* `psa_load_persistent_key` - reads attr.id and passes pointer to psa_parse_key_data_from_storage
-* `psa_parse_key_data_from_storage` - writes to many attributes
-* `psa_get_and_lock_key_slot` - writes to attr.id, attr.lifetime, and attr.policy.usage
-* `psa_purge_key` - reads attr.lifetime, calls psa_wipe_key_slot
-* `mbedtls_psa_get_stats` - reads attr.lifetime, attr.id
-
-`psa_crypto.c`:
-* `psa_get_and_lock_key_slot_with_policy` - reads attr.type, attr.policy.
-* `psa_get_and_lock_transparent_key_slot_with_policy` - reads attr.lifetime
-* `psa_destroy_key` - reads attr.lifetime, attr.id
-* `psa_get_key_attributes` - copies all publicly available attributes of a key
-* `psa_export_key` - copies attributes
-* `psa_export_public_key` - reads attr.type, copies attributes
-* `psa_start_key_creation` - writes to the whole attr structure 
-* `psa_validate_optional_attributes` - reads attr.type, attr.bits
-* `psa_import_key` - reads attr.bits
-* `psa_copy_key` - reads attr.bits, attr.type, attr.lifetime, attr.policy
-* `psa_mac_setup` - copies whole attr structure
-* `psa_mac_compute_internal` - copies whole attr structure
-* `psa_verify_internal` - copies whole attr structure
-* `psa_sign_internal` - copies whole attr structure, reads attr.type
-* `psa_assymmetric_encrypt` - reads attr.type
-* `psa_assymetric_decrypt` - reads attr.type
-* `psa_cipher_setup` - copies whole attr structure, reads attr.type
-* `psa_cipher_encrypt` - copies whole attr structure, reads attr.type
-* `psa_cipher_decrypt` - copies whole attr structure, reads attr.type
-* `psa_aead_encrypt` - copies whole attr structure
-* `psa_aead_decrypt` - copies whole attr structure
-* `psa_aead_setup` - copies whole attr structure
-* `psa_generate_derived_key_internal` - reads attr.type, writes to and reads from attr.bits, copies whole attr structure
-* `psa_key_derivation_input_key` - reads attr.type
-* `psa_key_agreement_raw_internal` - reads attr.type and attr.bits
-
-##### Determining whether a key slot is occupied
-
-`psa_is_key_slot_occupied` currently uses the `attr.type` field to determine whether a key slot is occupied. This works because we maintain the invariant that an occupied slot contains key material. With concurrency, it is desirable to allow a key slot to be reserved, but not yet contain key material or even metadata. When creating a key, determining the key type can be costly, for example when loading a persistent key from storage or (not yet implemented) when importing or unwrapping a key using an interface that determines the key type from the data that it parses. So we should not need to hold the global key store lock while the key type is undetermined.
-
-Instead, `psa_is_key_slot_occupied` should use the key identifier to decide whether a slot is occupied. The key identifier is always readily available: when allocating a slot for a persistent key, it's an input of the function that allocates the key slot; when allocating a slot for a volatile key, the identifier is calculated from the choice of slot.
-
-Alternatively, we could use a dedicated indicator that the slot is occupied. The advantage of this is that no field of the `attr` structure would be needed to determine the slot state. This would be a clean separation between key attributes and slot state and `attr` could be treated exactly like key slot content. This would save code size and maintenance effort. The cost of it would be that each slot would need an extra field to indicate whether it is occupied.
-
-##### Key slot content
-
-Other than what is used to determine the [“key slot state”](#key-slot-state), the contents of a key slot are only accessed as follows:
-
-* Modification during key creation (between `psa_start_key_creation` and `psa_finish_key_creation` or `psa_fail_key_creation`).
-* Destruction in `psa_wipe_key_slot`.
-* Read in many functions, between calls to `psa_lock_key_slot` and `psa_unlock_key_slot`.
-
-**slot->key access:** 
-* `psa_allocate_buffer_to_slot` - allocates key.data, sets key.bytes;
-* `psa_copy_key_material_into_slot` - writes to key.data
-* `psa_remove_key_data_from_memory` - writes and reads to/from key data
-* `psa_get_key_attributes` - reads from key data
-* `psa_export_key` - passes key data to psa_driver_wrapper_export_key
-* `psa_export_public_key` - passes key data to psa_driver_wrapper_export_public_key
-* `psa_finish_key_creation` - passes key data to psa_save_persistent_key
-* `psa_validate_optional_attributes` - passes key data and bytes to mbedtls_psa_rsa_load_representation
-* `psa_import_key` - passes key data to psa_driver_wrapper_import_key
-* `psa_copy_key` - passes key data to psa_driver_wrapper_copy_key, psa_copy_key_material_into_slot
-* `psa_mac_setup` - passes key data to psa_driver_wrapper_mac_sign_setup, psa_driver_wrapper_mac_verify_setup
-* `psa_mac_compute_internal` - passes key data to psa_driver_wrapper_mac_compute
-* `psa_sign_internal` - passes key data to psa_driver_wrapper_sign_message, psa_driver_wrapper_sign_hash
-* `psa_verify_internal` - passes key data to psa_driver_wrapper_verify_message, psa_driver_wrapper_verify_hash
-* `psa_asymmetric_encrypt` - passes key data to mbedtls_psa_rsa_load_representation
-* `psa_asymmetric_decrypt` - passes key data to mbedtls_psa_rsa_load_representation
-* `psa_cipher_setup ` - passes key data to psa_driver_wrapper_cipher_encrypt_setup and psa_driver_wrapper_cipher_decrypt_setup
-* `psa_cipher_encrypt` - passes key data to psa_driver_wrapper_cipher_encrypt
-* `psa_cipher_decrypt` - passes key data to psa_driver_wrapper_cipher_decrypt
-* `psa_aead_encrypt` - passes key data to psa_driver_wrapper_aead_encrypt
-* `psa_aead_decrypt` - passes key data to psa_driver_wrapper_aead_decrypt
-* `psa_aead_setup` - passes key data to psa_driver_wrapper_aead_encrypt_setup and psa_driver_wrapper_aead_decrypt_setup
-* `psa_generate_derived_key_internal` - passes key data to psa_driver_wrapper_import_key
-* `psa_key_derivation_input_key` - passes key data to psa_key_derivation_input_internal
-* `psa_key_agreement_raw_internal` - passes key data to mbedtls_psa_ecp_load_representation
-* `psa_generate_key` - passes key data to psa_driver_wrapper_generate_key
-
-#### Random generator
+### Random number generators
 
 The PSA RNG can be accessed both from various PSA functions, and from application code via `mbedtls_psa_get_random`.
 
-With the built-in RNG implementations using `mbedtls_ctr_drbg_context` or `mbedtls_hmac_drbg_context`, querying the RNG with `mbedtls_xxx_drbg_random()` is thread-safe (protected by a mutex inside the RNG implementation), but other operations (init, free, seed) are not.
+When using the built-in RNG implementations, i.e. when `MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG` is disabled, querying the RNG is thread-safe (`mbedtls_psa_random_init` and `mbedtls_psa_random_seed` are only thread-safe when called while holding `mbedtls_threading_psa_rngdata_mutex`. `mbedtls_psa_random_free` is not thread-safe).
 
-When `MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG` is enabled, thread safety depends on the implementation.
+When `MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG` is enabled, it is down to the external implementation to ensure thread-safety, should threading be enabled.
 
-#### Driver resources
+## Usage guide
 
-Depends on the driver. The PSA driver interface specification does not discuss whether drivers must support concurrent calls.
+### Initialization
 
-### Simple global lock strategy
+The PSA subsystem is initialized via a call to [`psa_crypto_init`](https://arm-software.github.io/psa-api/crypto/1.1/api/library/library.html#c.psa_crypto_init). This is a thread-safe function, and multiple calls to `psa_crypto_init` are explicitly allowed. It is valid to have multiple threads each calling `psa_crypto_init` followed by a call to any PSA key management function (if the init succeeds).
 
-Have a single mutex protecting all accesses to the key store and other global variables. In practice, this means every PSA API function needs to take the lock on entry and release on exit, except for:
+### General usage
 
-* Hash function.
-* Accessors for key attributes and other local structures.
+Once initialized, threads can use any PSA function if there is no overlap between their calls. All threads share the same set of keys, as soon as one thread returns from creating/loading a key via a key management API call the key can be used by any thread. If multiple threads attempt to load the same persistent key, with the same key identifier, only one thread can succeed - the others will return `PSA_ERROR_ALREADY_EXISTS`.
 
-Note that operation functions do need to take the lock, since they need to prevent the destruction of the key.
+Applications may need careful handling of resource management errors. As explained in ([PSA Concurrent calling conventions](#psa-concurrent-calling-conventions)), operations in progress can have memory related side effects. It is possible for a lack of resources to cause errors which do not arise in sequential execution. For example, multiple threads attempting to load the same persistent key can lead to some threads returning `PSA_ERROR_INSUFFICIENT_MEMORY` if the key is not currently in the key store - while trying to load a persistent key into the key store a thread temporarily reserves a free key slot.
 
-Note that this does not protect access to the RNG via `mbedtls_psa_get_random`, which is guaranteed to be thread-safe when `MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG` is disabled.
+If a mutex operation fails, which only happens if the mutex implementation fails, the error code `PSA_ERROR_SERVICE_FAILURE` will be returned. If this code is returned, execution of the PSA subsystem must be stopped. All functions which have internal mutex locks and unlocks (except for when the lock/unlock occurs in a function that has no return value) will return with this error code in this situation.
 
-This approach is conceptually simple, but requires extra instrumentation to every function and has bad performance in a multithreaded environment since a slow operation in one thread blocks unrelated operations on other threads.
+### Freeing
 
-### Global lock excluding slot content
+There is no thread-safe way to free all PSA resources. This is because any such operation would need to wait for all other threads to complete their tasks before wiping resources.
 
-Have a single mutex protecting all accesses to the key store and other global variables, except that it's ok to access the content of a key slot without taking the lock if one of the following conditions holds:
+`mbedtls_psa_crypto_free` must only be called by a single thread once all threads have completed their operations.
 
-* The key slot is in a state that guarantees that the thread has exclusive access.
-* The key slot is in a state that guarantees that no other thread can modify the slot content, and the accessing thread is only reading the slot.
+## Current strategy
 
-Note that a thread must hold the global mutex when it reads or changes a slot's state.
+This section describes how we have implemented thread-safety. There is discussion of: techniques, internal properties for enforcing thread-safe access, how the system stays consistent and our abstraction model.
 
-#### Slot states
+### Protected resources
 
-For concurrency purposes, a slot can be in one of four states:
+#### Global data
 
-* EMPTY: no thread is currently accessing the slot, and no information is stored in the slot. Any thread is able to change the slot's state to FILLING and begin loading data.
-* FILLING: one thread is currently loading or creating material to fill the slot, this thread is responsible for the next state transition. Other threads cannot read the contents of a slot which is in FILLING.
-* FULL: the slot contains a key, and any thread is able to use the key after registering as a reader.
-* PENDING_DELETION: the key within the slot has been destroyed or marked for destruction, but at least one thread is still registered as a reader. No thread can register to read this slot. The slot must not be wiped until the last reader de-registers, wiping the slot by calling `psa_wipe_key_slot`.
+We have added a mutex `mbedtls_threading_psa_globaldata_mutex` defined in `include/mbedtls/threading.h`, which is used to make `psa_crypto_init` thread-safe.
 
-To change `slot` to state `new_state`, a function must call `psa_slot_state_transition(slot, new_state)`.
+There are two `psa_global_data_t` structs, each with a single instance `global_data`:
 
-A counter field within each slot keeps track of how many readers have registered. Library functions must call `psa_register_read` before reading the key data within a slot, and `psa_unregister_read` after they have finished operating.
+* The struct in `library/psa_crypto.c` is protected by `mbedtls_threading_psa_globaldata_mutex`. The RNG fields within this struct are not protected by this mutex, and are not always thread-safe (see [Random number generators](#random-number-generators)).
+* The struct in `library/psa_crypto_slot_management.c` has two fields: `key_slots` is protected as described in [Key slots](#key-slots), `key_slots_initialized` is protected by the global data mutex.
 
-Any call to `psa_slot_state_transition`, `psa_register_read` or `psa_unregister_read` must be performed by a thread which holds the global mutex.
+#### Mutex usage
 
-##### Linearizability of the system
+A deadlock would occur if a thread attempts to lock a mutex while already holding it. Functions which need to be called while holding the global mutex have documentation to say this.
+
+To avoid performance degradation, functions must hold mutexes for as short a time as possible. In particular, they must not start expensive operations (eg. doing cryptography) while holding the mutex.
+
+#### Key slots
+
+
+Keys are stored internally in a global array of key slots known as the "key store", defined in `library/psa_slot_management.c`.
+
+##### Key slot states
+
+Each key slot has a state variable and a `registered_readers` counter. These two variables dictate whether an operation can access a slot, and in what way the slot can be used.
+
+There are four possible states for a key slot:
+
+* `PSA_SLOT_EMPTY`: no thread is currently accessing the slot, and no information is stored in the slot. Any thread is able to change the slot's state to `PSA_SLOT_FILLING` and begin to load data into the slot.
+* `PSA_SLOT_FILLING`: one thread is currently loading or creating material to fill the slot, this thread is responsible for the next state transition. Other threads cannot read the contents of a slot which is in this state.
+* `PSA_SLOT_FULL`: the slot contains a key, and any thread is able to use the key after registering as a reader, increasing `registered_readers` by 1.
+* `PSA_SLOT_PENDING_DELETION`: the key within the slot has been destroyed or marked for destruction, but at least one thread is still registered as a reader (`registered_readers > 0`). No thread can register to read this slot. The slot must not be wiped until the last reader unregisters. It is during the last unregister that the contents of the slot are wiped, and the slot's state is set to `PSA_SLOT_EMPTY`.
+
+###### Key slot state transition diagram
+![](key-slot-state-transitions.png)
+
+In the state transition diagram above, an arrow between two states `q1` and `q2` with label `f` indicates that if the state of a slot is `q1` immediately before `f`'s linearization point, it may be `q2` immediately after `f`'s linearization point. Internal functions have italicized labels. The `PSA_SLOT_PENDING_DELETION -> PSA_SLOT_EMPTY` transition can be done by any function which calls `psa_unregister_read`.
+
+The state transition diagram can be generated in https://app.diagrams.net/ via this [url](https://viewer.diagrams.net/?tags=%7B%7D&highlight=0000ff&edit=_blank&layers=1&nav=1#R3Vxbd5s4EP4t%2B%2BDH5CBxf6zrJJvW7aYn7W7dFx9qZFstBg7gW379CnMxkoUtY%2BGQ%2BiVISCPQjD59mhnSU98vNg%2BRE84%2FBS7yelBxNz110IMQAEsnf9KabVZjmHnFLMJu3mhf8YxfUF6p5LVL7KKYapgEgZfgkK6cBL6PJglV50RRsKabTQOPHjV0Zuig4nnieIe1%2F2E3mWe1FjT39X8jPJsXIwPDzu4snKJx%2Fibx3HGDdaVKveup76MgSLKrxeY98tLJK%2BYl63dfc7d8sAj5iUiHH%2BBlOP338cP6i%2B37%2Ff7oV%2Fjr442aSVk53jJ%2F4R40PCKv7%2BIVuZyll%2FffhsOimsiv3OE0njvxOEKOi6K4uPszYtuzUnbzk2yLSScPTvRLCv31HCfoOXQm6Z01MbF0hGThkRIgl04cZkqf4g1yS1HVScnnaYWiBG0qVfkkPaBggZJoS5rkdzUrV1hhsUpeXlf0n1fNK6ov6pzc4mal5L1SyEWulzN0BABHSeyM%2Be671NpJaeI5cYwn9ERFwdJ30xkaKKREJifafs9v7QqjamGwqbYbbIvSBidlJ3I9qtTvu6SFoketNuJgGU3QabtMnGiGkiPttKwdcqlVfKjbiu50ju6Kugh5ToJX9NrnKTQf4SnA5M1qTUc3GJvI3jvvVV2rrCDTvrUrP4sSq6mM2GyaDsTurK2chAsMENaiBC7WcBg746UfoRmOExTtEKCy2HH9UieaGzo%2Fya5BL2wPz%2FzUmInloIhUpOsXE1h%2Bl99YYNdNZfQjFOMX5%2BdOXmpzYToLu3nR%2Bz19wLXC48uMRYpyc8lHofCbhyDKLVRMm1LZDbzMwAoxgOkSTKcxakfpIjvD3aenr6O3CfOdQ3lbOsrneK1U8BocxetyXygLo2qhZl9ojvJQEOVBt1CetpwDNBYG%2BRObRcuoXvDSU6g%2BdbA3%2Fo224wkB9QQH%2FlvD9WJhdRHXc8mQEsr2bw%2FkDzf2%2B8fh8PHzQ6exWjVeGas1kb3xrFPTX3%2FcsenVlaSLKOnp7vNgZ%2B6CehrcDe%2B%2BPv7z%2BW3qqHOkx2yL84ifUZudhZtznsKJdYrzwE5xHqiQzc%2FSoAnI2VTTDXoX1DXj1gS6CS1TJwWVES9KiIDBMCvtuozIEkEMLkciZAVFKzSeRgjtuFLsBQmfJwkCDXeYmExAwuViXBw6OWpnOVuBC12kbKUY7VosDfD4hnyYvNWbHA6zXq96POyWEzCFSkUpoNIgqEaDGkhdewVWqpZiNgNLTWHAkti6yphk237B5oA5xT6O5wLHyjcGXOVSvRi5bogVabZJQ5cqx0ItrtQrABmPkzO6nCzJRuqWFOx6YQ1xN1lzRBMNa6idQjStiNmWMdyGHi%2FdYASxB4sawCI24GwrzfLlWf%2FANo2NpqIcfy7ItAcn2mvWMfnkInvipotn0NcmAD9MQu8FLR%2Fxs%2F7uaSN2nq1hpyejMpew0pqwTzNKKjYkMZKx47tjL5j8Lvn2%2BPtFA6VyJ14Q7wj8Wb3CJbHaaq%2BDwf8wel7iuIxdDqgWvZou5Oe5ZJr0Q%2F1ae5zKS6mQQtarG5SgT6PCztuN5GiCG1u3IjnQhJSV6HrDjQ3UOdauxMRV3gmRi1UuipMo2F6OcXLwtLMQVy5jCS4IzTLoM2CxDC403xuaTdktQByXicj32nKJ%2Bym0Oh8X28e3bnltVYbX6k1D1arJOBsEibssi6t3NDR1w3YBeI4uLinUymYc9ZJwBxRujjY9CNzZuUqSjLAnlIarFj2hon4DvdPwY4Cm8MOkyhjtJUByra547orZHXCpzgKKtPSXFFCKrpKJDO3mbCP9ha%2FXK2VWn4aGJjDUHE50QTjp2Gmtxkt3NpxAhs0Y7WXe8c0O1tKZhr42eZ61NQ4PqdPbdV8dX%2FYywsvlF05yIRGorwSJPKrNaFJ6iKaxX6oryMTEGxoHSFTNvIWWpWtQszUbqpbKyqVCy1AIts6NnpC3qY4CbPohTEW9NaFS%2FtTjbwTso8IAOEeY3vzJ2gnKcLP23%2FKnMcdBQQJgKrpFc0hJFLKNbJwnvNwMp3BsWbMvqx%2F3Hye%2BH3I%2FjJHDGanEmkZf47XGGEWzFruViqMyOTI667YSxmX9hCNNHmPk2pwQYUxxBi%2FCIEsRPMtPP0M%2BipykgYM%2FCM%2BPJaT00kURXu3yfsbBMgmX1DOfn1X9GlB5FB0kIKWuAe65%2BGLvHSX0almMsLMJDCeyCeScfv6wT%2FdEAyKimUz7YFkRebtSbpNNu7IPcs6F8zEZQaIh4L0gqUvww0j7vh7F%2FW9ujL7iR%2FfmYWy1QF0KOy2JxzmWSicnvP4nF93KumPJi9n4UMmQFxOKWea550bW3W9qcrPiuCZdz4yaJ4x1gVwcXb8SyAWwDTlsQmUijIxPogmYkeL%2B3%2BJkzff%2FXEi9%2Bx8%3D).
+##### Key slot access primitives
+
+The state of a key slot is updated via the internal function `psa_key_slot_state_transition`. To change the state of `slot` from `expected_state` to `new_state`, when `new_state` is not `PSA_SLOT_EMPTY`, one must call `psa_key_slot_state_transition(slot, expected_state, new_state)`; if the state was not `expected_state` then `PSA_ERROR_CORRUPTION_DETECTED` is returned. The sole reason for having an expected state parameter here is to help guarantee that our functions work as expected, this error code cannot occur without an internal coding error.
+
+Changing a slot's state to `PSA_SLOT_EMPTY` is done via `psa_wipe_key_slot`, this function wipes the entirety of the key slot.
+
+The reader count of a slot is incremented via `psa_register_read`, and decremented via `psa_unregister_read`. Library functions register to read a slot via the `psa_get_and_lock_key_slot_X` functions, read from the slot, then call `psa_unregister_read` to make known that they have finished reading the slot's contents.
+
+##### Key store consistency and abstraction function
+
+The key store is protected by a single global mutex `mbedtls_threading_key_slot_mutex`.
+
+We maintain the consistency of the key store by ensuring that all reads and writes to `slot->state` and `slot->registered_readers` are performed under `mbedtls_threading_key_slot_mutex`. All the access primitives described above must be called while the mutex is held; there is a convenience function `psa_unregister_read_under_mutex` which wraps a call to `psa_unregister_read` in a mutex lock/unlock pair.
+
+A thread can only traverse the key store while holding `mbedtls_threading_key_slot_mutex`, the set of keys within the key store which the thread holding the mutex can access is equivalent to the set:
+
+    {mbedtls_svc_key_id_t k : (\exists slot := &global_data.key_slots[i]) [
+                                  (slot->state == PSA_SLOT_FULL) &&
+                                  (slot->attr.id == k)]}
+
+The union of this set and the set of persistent keys not currently loaded into slots is our abstraction function for the key store, any key not in this union does not currently exist as far as the code is concerned (even if the key is in a slot which has a `PSA_SLOT_FILLING` or `PSA_SLOT_PENDING_DELETION` state). Attempting to start using any key which is not a member of the union will result in a `PSA_ERROR_INVALID_HANDLE` error code.
+
+##### Locking and unlocking the mutex
+
+If a lock or unlock operation fails and this is the first failure within a function, the function will return `PSA_ERROR_SERVICE_FAILURE`. If a lock or unlock operation fails after a different failure has been identified, the status code is not overwritten.
+
+We have defined a set of macros in `library/psa_crypto_core.h` to capture the common pattern of (un)locking the mutex and returning or jumping to an exit label upon failure.
+
+##### Key creation and loading
+
+To load a new key into a slot, the following internal utility functions are used:
+
+* `psa_reserve_free_key_slot` - This function, which must be called under `mbedtls_threading_key_slot_mutex`, iterates through the key store to find a slot whose state is `PSA_SLOT_EMPTY`. If found, it reserves the slot by setting its state to `PSA_SLOT_FILLING`. If not found, it will see if there are any persistent keys loaded which do not have any readers, if there are it will kick one such key out of the key store.
+* `psa_start_key_creation` - This function wraps around `psa_reserve_free_key_slot`, if a slot has been found then the slot id is set. This second step is not done under the mutex, at this point the calling thread has exclusive access to the slot.
+* `psa_finish_key_creation` - After the contents of the key have been loaded (again this loading is not done under the mutex), the thread calls `psa_finish_key_creation`. This function takes the mutex, checks that the key does not exist in the key store (this check cannot be done before this stage), sets the slot's state to `PSA_SLOT_FULL` and releases the mutex. Upon success, any thread is immediately able to use the new key.
+* `psa_fail_key_creation` - If there is a failure at any point in the key creation stage, this clean-up function takes the mutex, wipes the slot, and releases the mutex. Immediately after this unlock, any thread can start to use the slot for another key load.
+
+##### Re-loading persistent keys
+
+As described above, persistent keys can be kicked out of the key slot array provided they are not currently being used (`registered_readers == 0`). When attempting to use a persistent key that has been kicked out of a slot, the call to `psa_get_and_lock_key_slot` will see that the key is not in a slot, call `psa_reserve_free_key_slot` and load the key back into the reserved slot. This entire sequence is done during a single mutex lock, which is necessary for thread-safety (see documentation of `psa_get_and_lock_key_slot`).
+
+If `psa_reserve_free_key_slot` cannot find a suitable slot, the key cannot be loaded back in. This will lead to a `PSA_ERROR_INSUFFICIENT_MEMORY` error.
+
+##### Using existing keys
+
+One-shot operations follow a standard pattern when using an existing key:
+
+* They call one of the `psa_get_and_lock_key_slot_X` functions, which then finds the key and registers the thread as a reader.
+* They operate on the key slot, usually copying the key into a separate buffer to be used by the operation. This step is not performed under the key slot mutex.
+* Once finished, they call `psa_unregister_read_under_mutex`.
+
+Multi-part and restartable operations each have a "setup" function where the key is passed in, these functions follow the above pattern. The key is copied into the `operation` object, and the thread unregisters from reading the key (the operations do not access the key slots again). The copy of the key will not be destroyed during a call to `psa_destroy_key`, the thread running the operation is responsible for deleting its copy in the clean-up. This may need to change to enforce the long term key requirements ([Long term key destruction requirements](#long-term-key-destruction-requirements)).
+
+##### Key destruction implementation
+
+The locking strategy here is explained in `library/psa_crypto.c`. The destroying thread (the thread calling `psa_destroy_key`) does not always wipe the key slot. The destroying thread registers to read the key, sets the slot's state to `PSA_SLOT_PENDING_DELETION`, wipes the slot from memory if the key is persistent, and then unregisters from reading the slot.
+
+`psa_unregister_read` internally calls `psa_wipe_key_slot` if and only if the slot's state is `PSA_SLOT_PENDING_DELETION` and the slot's registered reader counter is equal to 1. This implements a "last one out closes the door" approach. The final thread to unregister from reading a destroyed key will automatically wipe the contents of the slot; no readers remain to reference the slot post deletion, so there cannot be corruption.
+
+### linearizability of the system
 
 To satisfy the requirements in [Correctness out of the box](#correctness-out-of-the-box), we require our functions to be "linearizable" (under certain constraints). This means that any (constraint satisfying) set of concurrent calls are performed as if they were executed in some sequential order.
 
 The standard way of reasoning that this is the case is to identify a "linearization point" for each call, this is a single execution step where the function takes effect (this is usually a step in which the effects of the call become visible to other threads). If every call has a linearization point, the set of calls is equivalent to sequentially performing the calls in order of when their linearization point occurred.
 
-We only require linearizability to hold in the case where a resource-management error is not returned. In a set of concurrent calls, it is permitted for a call c to fail with a PSA_ERROR_INSUFFICIENT_MEMORY return code even if there does not exist a sequential ordering of the calls in which c returns this error. Even if such an error occurs, all calls are still required to be functionally correct.
+We only require linearizability to hold in the case where a resource-management error is not returned. In a set of concurrent calls, it is permitted for a call c to fail with a `PSA_ERROR_INSUFFICIENT_MEMORY` return code even if there does not exist a sequential ordering of the calls in which c returns this error. Even if such an error occurs, all calls are still required to be functionally correct.
 
-We only access and modify a slot's state and reader count while we hold the global lock. This ensures the memory in which these fields are stored is correctly synchronized. It also ensures that the key data within the slot is synchronised where needed (the writer unlocks the mutex after filling the data, and any reader must lock the mutex before reading the data).
+To help justify that our system is linearizable, here are the linearization points/planned linearization points of each PSA call :
 
-To help justify that our system is linearizable, here is a list of key slot state changing functions and their linearization points (for the sake of brevity not all failure cases are covered, but those cases are not complex):
-* `psa_wipe_key_slot, psa_register_read, psa_unregister_read, psa_slot_state_transition,` - These functions are all always performed under the global mutex, so they have no effects visible to other threads (this implies that they are linearizable).
-* `psa_get_empty_key_slot, psa_get_and_lock_key_slot_in_memory, psa_load_X_key_into_slot, psa_fail_key_creation` - These functions hold the mutex for all non-setup/finalizing code, their linearization points are the release of the mutex.
-* `psa_get_and_lock_key_slot` - If the key is already in a slot, the linearization point is the linearization point of the call to `psa_get_and_lock_key_slot_in_memory`. If the key is not in a slot and is loaded into one, the linearization point is the linearization point of the call to `psa_load_X_key_into_slot`.
-* `psa_start_key_creation` - From the perspective of other threads, the only effect of a successful call to this function is that the amount of usable resources decreases (a key slot which was usable is now unusable). Since we do not consider resource management as linearizable behaviour, when arguing for linearizability of the system we consider this function to have no visible effect to other threads.
-* `psa_finish_key_creation` - On a successful load, we lock the mutex and set the state of the slot to FULL, the linearization point is then the following unlock. On an unsuccessful load, the linearization point is when we return - no action we have performed has been made visible to another thread as the slot is still in a FILLING state.
-* `psa_destroy_key, psa_close_key, psa_purge_key` - As per the requirements, we need only argue for the case where the key is not in use here. The linearization point is the unlock after wiping the data and setting the slot state to EMPTY.
-* `psa_import_key, psa_copy_key, psa_generate_key, mbedtls_psa_register_se_key` - These functions call both `psa_start_key_creation` and `psa_finish_key_creation`, the linearization point of a successful call is the linearization point of the call to `psa_finish_key_creation`. The linearization point of an unsuccessful call is the linearization point of the call to `psa_fail_key_creation`.
-* `psa_key_derivation_output_key` - Same as above. If the operation object is in use by multiple threads, the behaviour need not be linearizable.
+* Key creation functions (including `psa_copy_key`) - The linearization point for a successful call is the mutex unlock within `psa_finish_key_creation`; it is at this point that the key becomes visible to other threads. The linearization point for a failed call is the closest mutex unlock after the failure is first identified.
+* `psa_destroy_key` - The linearization point for a successful destruction is the mutex unlock, the slot is now in the state `PSA_SLOT_PENDING_DELETION` meaning that the key has been destroyed. For failures, the linearization point is the same.
+* `psa_purge_key`, `psa_close_key` - The linearization point is the mutex unlock after wiping the slot for a success, or unregistering for a failure.
+* One shot operations - The linearization point is the final unlock of the mutex within `psa_get_and_lock_key_slot`, as that is the point in which it is decided whether or not the key exists.
+* Multi-part operations - The linearization point of the key input function is the final unlock of the mutex within `psa_get_and_lock_key_slot`. All other steps have no non resource-related side effects (except for key derivation, covered in the key creation functions).
 
-Library functions which operate on a slot will return `PSA_ERROR_BAD_STATE` if the slot is in an inappropriate state for the function at the linearization point.
+Please note that one shot operations and multi-part operations are not yet considered thread-safe, as we have not yet tested whether they rely on unprotected global resources. The key slot access in these operations is thread-safe.
 
-##### Key slot state transition diagram
+## Testing and analysis
 
-![](key-slot-state-transitions.png)
+### Thread-safe testing
 
-In the state transition diagram above, an arrow between two states `q1` and `q2` with label `f` indicates that if the state of a slot is `q1` immediately before `f`'s linearization point, it may be `q2` immediately after `f`'s linearization point.
+It is now possible for individual tests to spin up multiple threads. This work has made the global variables used in tests thread-safe. If multiple threads fail a test assert, the first failure will be reported with correct line numbers.
 
-##### Generating the key slot state transition diagram from source
+Although the `step` feature used in some tests is thread-safe, it may produce unexpected results for multi-threaded tests. `mbedtls_test_set_step` or `mbedtls_test_increment_step` calls within threads can happen in any order, thus may not produce the desired result when precise ordering is required.
 
-To generate the state transition diagram in https://app.diagrams.net/, open the following url:
+### Current state of testing
 
-https://viewer.diagrams.net/?tags=%7B%7D&highlight=FFFFFF&edit=_blank&layers=1&nav=1&title=key-slot-state-transitions#R5Vxbd5s4EP4t%2B%2BDH5iAJcXms4ySbrdtNT7qX9MWHgGyrxcABHNv59SsM2EhgDBhs3PVL0CANoBl9fDMaMkC3i%2FWDb3jzz65F7AGUrPUAjQYQAqBh9ieSbGKJIqFYMPOplXTaC57pO0mEUiJdUosEXMfQde2QerzQdB2HmCEnM3zfXfHdpq7NX9UzZiQneDYNOy%2F9h1rhPJZqUN3Lfyd0Nk%2BvDBQ9PrMw0s7JkwRzw3JXGRG6G6Bb33XD%2BGixviV2NHnpvMTj7g%2Bc3d2YT5ywyoDv4H08%2Ffvxj9VX3XGGw5cf3o9PHxJjvBn2MnngAVRspm9o0Td2OIsO7%2F8aj1Mx0585U9B5bgQTnxgW8YP07Ksv9he1bOcn3KSTzm6c2Zc1hqs5DcmzZ5jRmRVzsegK4cJmLcAOjcCLjT6la2LtVGUnJZmnN%2BKHZJ0RJZP0QNwFCf0N65KclbXEYDuPTdqrjP0T0Txj%2BlRmJB4322neG4UdJHapYSMACowkzphjfYy8nbVM2wgCavIT5btLx4pmaCSxFpscf%2FNvcmrbeMk2Rutsv9Emba1puBvEjl8y8v2QqJGOOGiNwF36Jjnul6Hhz0hY0k%2BO%2BxGLW8V522Zshwtsl8p8YhshfePXfpFBkys8uZQ92UHXwYrgE%2FFzJ6Oya1VUpOo3euancWplJKiNpymnduttu0k4wQFhzgGXjk9mNAiJv13seX9kBhkbr%2BxlwK9Xm86cyEeZQxCfCaJlSRnafkxOLKhlRTqGPgnou%2FG61Re5khc93PZx8XCAR4XOVb56RADYvTOSq3CwXAQM0g2UVJ2zxAd4mt%2BkaoAwxJ1OA9KNLasA%2Ft3np28v14nevQNvvXXwTmBYysAwKIXhHdxLWbiXjsB9c%2FCGFcEb9Au8ec%2FJgWxl7D7yDugYrFO6mXE4LzAmU4Pak59kMzEZXofUdfoM2ema6SNkJ5ohp1Qc3x1%2B51%2FF94%2Fj8eOXh17DMFIuDMNyldderTjnt18u0Lm4kXAVIz3dfRlt3b2inUZ347tvj39%2BuU4b9Y7PqF3RmepRZbPotTmdSdNOx%2BgM7BWdgRJ7%2BWkyVAGLJmWs8G9BLCs3KsAq1FTMGkhQX5XrAEUgTfJ5yY5WyHXYFSdk4YWbLeEJbDfsMdlJF1Qfuc5OjXwuegOKXtTt48sNbhIwxaMuGjL1K98VYYwkpRijMDjg0QBEWawUZJAmqc1QRpYElGG%2BjgSX7DoFVow0U%2BrQYH41cVW6uE7Gmg%2FM7rKu8mCDWvEpRSvUegboKaKfgi3Npf%2B2RZaYbZwv51492dMcg6rm3FGvMEhWMecwitowb4MVQZHIoQ9ADPMBY5PplizPwzes82imSlL5fUGhPzjSX9bK9LOD%2BI6bLp7RUDYBfTA9%2B50sH%2Bkz%2Fvi0rha6CVsGFQO4lNEZjjWxXfNnhtTV0GDabkCiobVGeUtm8uyo%2BtFjf9A%2FtVEb6A%2BQxntZO1k1nr5CfC7sR0X74K3QzixwVwxrMzyz2zy9XBHw%2B5WnhyrkvATjhoAPDuVWzsQpUVGsUwhDFglC392cDl%2FtQGVvIW63jFsIpmVN4aOZdBmc6L47HN5wkNc9xsmX4LfHwKs%2BTB6Eu57AE6N3mcwa0gBnbaSCorO1uaqsZpJ7CtDrXKQjHouQVn7P4l2iIzwWl%2BrvhsfmyyOup9JFbo3gsegeC47bEvh1kUgsNGT7%2BxSXxrfW6BzsFV4iIbzFTesukCpkCSvG72153HXtRZQumlYiRF3YcmqLPqVZzC4ThIWzc5ZKrspbEzwMdbg1UTUtiHsNKwpoCitCPZfSXfFtMSMprufiQsLeAkprhVwRoECekbQVj%2FG7GF0UchXb9UxV%2FcehoQkMNYcTXBFO%2BhXVwQNJ%2BNpwAgWWonRXHlrsdrDA7XJpoFzQUyN9tKIeyeXoryNvXr5Q26jQ2H0P1y6IAXQhEMuT3pwlz55TOohNfcESIXHSeMcSbbNAGpahrMs6RBoS9XLVGbAS0NRNA7GnyV4F6PxNqBK6UaG0%2B6HyJwJ6qTIA6ijDze%2Bso%2BxSPoToZXqpfK3%2Fz9JLT3S5Hk%2FhRNNmX9%2B%2B338yHccr%2FIyqHfLGlZw1%2BiSzM%2BpWtRC2X0VqSKgew2JeqDLc4iOZqvaoW6HPVWJuEQOzXcOaeMQPIlxxwi0ZY%2Ffk1q%2Ba2Gp6XVI7pM4JakrLN66DGpaiQAuIiGVQGIie6Pxnq6CAl6wAqu9Cv9gXl1VT%2F1VL9%2Fa74OmW%2Brk2T%2Fnkbu57gsolw4KiqrUde0WnLBnW3P9fj7j7%2Fr%2BjoLv%2FAA%3D%3D
+Our testing is a work in progress. It is not feasible to run our traditional, single-threaded, tests in such a way that tests concurrency. We need to write new test suites for concurrency testing.
 
-#### Destruction of a key in use
+Our tests currently only run on pthread, we hope to expand this in the future (our API already allows this).
 
-Problem: In [Key destruction long-term requirements](#key-destruction-long-term-requirements) we require that the key slot is destroyed (by `psa_wipe_key_slot`) even while it's in use (FILLING or with at least one reader).
+We run tests using [ThreadSanitizer](https://clang.llvm.org/docs/ThreadSanitizer.html) to detect data races. We test the key store, and test that our key slot state system is enforced. We also test the thread-safety of `psa_crypto_init`.
 
-How do we ensure that? This needs something more sophisticated than mutexes (concurrency number >2)! Even a per-slot mutex isn't enough (we'd need a reader-writer lock).
+Currently, not every API call is tested, we also cannot feasibly test every combination of concurrent API calls. API calls can in general be split into a few categories, each category calling the same internal key management functions in the same order - it is the internal functions that are in charge of locking mutexes and interacting with the key store; we test the thread-safety of these functions.
 
-Solution: after some team discussion, we've decided to rely on a new threading abstraction which mimics C11 (i.e. `mbedtls_fff` where `fff` is the C11 function name, having the same parameters and return type, with default implementations for C11, pthreads and Windows). We'll likely use condition variables in addition to mutexes.
+Since we do not run every cryptographic operation concurrently, we do not test that operations are free of unexpected global variables.
 
-##### Mutex only
+### Expanding testing
 
-When calling `psa_wipe_key_slot` it is the callers responsibility to set the slot state to PENDING_DELETION first. For most functions this is a clean {FULL, !has_readers} -> PENDING_DELETION transition: psa_get_empty_key_slot, psa_get_and_lock_key_slot, psa_close_key, psa_purge_key.
+Through future work on testing, it would be good to:
 
-`psa_wipe_all_key_slots` is only called from `mbedtls_psa_crypto_free`, here we will need to return an error as we won't be able to free the key store if a key is in use without compromising the state of the secure side. This is acceptable as an untrusted application cannot call `mbedtls_psa_crypto_free` in a crypto service. In a service integration, `mbedtls_psa_crypto_free` on the client cuts the communication with the crypto service. Also, this is the current behaviour.
+* For every API call, have a test which runs multiple copies of the call simultaneously.
+* After implementing other threading platforms, expand the tests to these platforms.
+* Have increased testing for kicking persistent keys out of slots.
+* Explicitly test that all global variables are protected, for this we would need to cover every operation in a concurrent scenario while running ThreadSanitizer.
+* Run tests on more threading implementations, once these implementations are supported.
 
-`psa_destroy_key` registers as a reader, marks the slot as deleted, deletes persistent keys and opaque keys and unregisters before returning. This will free the key ID, but the slot might be still in use. This only works if drivers are protected by a mutex (and the persistent storage as well if needed). `psa_destroy_key` transfers to PENDING_DELETION as an intermediate state. The last reading operation will wipe the key slot upon unregistering. In case of volatile keys freeing up the ID while the slot is still in use does not provide any benefit and we don't need to do it.
+### Performance
 
-These are serious limitations, but this can be implemented with mutexes only and arguably satisfies the [Key destruction short-term requirements](#key-destruction-short-term-requirements).
+Key loading does somewhat run in parallel, deriving the key and copying it key into the slot is not done under any mutex.
 
-Variations:
+Key destruction is entirely sequential, this is required for persistent keys to stop issues with re-loading keys which cannot otherwise be avoided without changing our approach to thread-safety.
 
-1. As a first step the multipart operations would lock the keys for reading on setup and release on free
-2. In a later stage this would be improved by locking the keys on entry into multi-part API calls and released before exiting.
 
-The second variant can't be implemented as a backward compatible improvement on the first as multipart operations that were successfully completed in the first case, would fail in the second. If we want to implement these incrementally, multipart operations in a multithreaded environment must be left unsupported in the first variant. This makes the first variant impractical (multipart operations returning an error in builds with multithreading enabled is not a behaviour that would be very useful to release).
+## Future work
 
-We can't reuse the `lock_count` field to mark key slots deleted, as we still need to keep track the lock count while the slot is marked for deletion. This means that we will need to add a new field to key slots. This new field can be reused to indicate whether the slot is occupied (see section [Determining whether a key slot is occupied](#determining-whether-a-key-slot-is-occupied)). (There would be three states: deleted, occupied, empty.)
+### Long term requirements
+
+As explained previously, we eventually aim to make the entirety of the PSA API thread-safe. This will build on the work that we have already completed. This requires a full suite of testing, see [Expanding testing](#expanding-testing) for details.
+
+### Long term performance requirements
+
+Our plan for cryptographic operations is that they are not performed under any global mutex. One-shot operations and multi-part operations will each only hold the global mutex for finding the relevant key in the key slot, and unregistering as a reader after the operation, using their own operation-specific mutexes to guard any shared data that they use.
+
+We aim to eventually replace some/all of the mutexes with RWLocks, if possible.
+
+### Long term key destruction requirements
+
+The [PSA Crypto Key destruction specification](https://arm-software.github.io/psa-api/crypto/1.1/api/keys/management.html#key-destruction) mandates that implementations make a best effort to ensure that the key material cannot be recovered. In the long term, it would be good to guarantee that `psa_destroy_key` wipes all copies of the key material.
+
+Here are our long term key destruction goals:
+
+`psa_destroy_key` does not block indefinitely, and when `psa_destroy_key` returns:
+
+1. The key identifier does not exist. This is a functional requirement for persistent keys: any thread can immediately create a new key with the same identifier.
+2. The resources from the key have been freed. This allows threads to create similar keys immediately after destruction, regardless of resources.
+4. No copy of the key material exists. Rationale: this is a security requirement. We do not have this requirement yet, but we need to document this as a security weakness, and we would like to satisfy this security requirement in the future.
 
 #### Condition variables
 
-Clean UNUSED -> PENDING_DELETION transition works as before.
+It would be ideal to add these to a future major version; we cannot add these as requirements to the default `MBEDTLS_THREADING_C` for backwards compatibility reasons.
 
-`psa_wipe_all_key_slots` and `psa_destroy_key` mark the slot as deleted and go to sleep until the slot has no registered readers. When waking up, they wipe the slot, and return.
+Condition variables would enable us to fulfil the final requirement in [Long term key destruction requirements](#long-term-key-destruction-requirements). Destruction would then work as follows:
 
-If the slot is already marked as deleted the threads calling `psa_wipe_all_key_slots` and `psa_destroy_key` go to sleep until the deletion completes. To satisfy [Key destruction long-term requirements](#key-destruction-long-term-requirements) none of the threads may return from the call until the slot is deleted completely. This can be achieved by signalling them when the slot has already been wiped and ready for use, that is not marked for deletion anymore. To handle spurious wake-ups, these threads need to be able to tell whether the slot was already deleted. This is not trivial, because by the time the thread wakes up, theoretically the slot might be in any state. It might have been reused and maybe even marked for deletion again.
+ * When a thread calls `psa_destroy_key`, they continue as normal until the `psa_unregister_read` call.
+ * Instead of calling `psa_unregister_read`, the thread waits until the condition `slot->registered_readers == 1` is true (the destroying thread is the final reader).
+ * At this point, the destroying thread directly calls `psa_wipe_key_slot`.
 
-To resolve this, we can either:
+A few changes are needed for this to follow our destruction requirements:
 
-1. Depend on the deletion marker. If the slot has been reused and is marked for deletion again, the threads keep waiting until the second deletion completes.
-2. Introduce a uuid (eg a global counter plus a slot ID), which is recorded by the thread waiting for deletion and checks whether it matches. If it doesn't, the function can return as the slot was already reallocated. If it does match, it can check whether it is still marked for deletion, if it is, the thread goes back to sleep, if it isn't, the function can return.
+ * Multi-part operations will need to remain registered as readers of their key slot until their copy of the key is destroyed, i.e. at the end of the finish/abort call.
+ * The functionality where `psa_unregister_read` can wipe the key slot will need to be removed, slot wiping is now only done by the destroying/wiping thread.
 
-##### Platform abstraction
+### Protecting operation contexts
 
-Introducing condition variables to the platform abstraction layer would be best done in a major version. If we can't wait until that, we will need to introduce a new compile time flag. Considering that this only will be needed on the PSA Crypto side and the upcoming split, it makes sense to make this flag responsible for the entire PSA Crypto threading support. Therefore if we want to keep the option open for implementing this in a backward compatible manner, we need to introduce and use this new flag already when implementing [Mutex only](#mutex-only). (If we keep the abstraction layer for mutexes the same, this shouldn't mean increase in code size and would mean only minimal effort on the porting side.)
+Currently, we rely on the crypto service to ensure that the same operation is not invoked concurrently. This abides by the PSA Crypto API Specification ([PSA Concurrent calling conventions](#psa-concurrent-calling-conventions)).
 
-#### Operation contexts
+Concurrent access to the same operation object can compromise the crypto service. For example, if the operation context has a pointer (depending on the compiler and the platform, the pointer assignment may or may not be atomic). This violates the functional correctness requirement of the crypto service.
 
-Concurrent access to the same operation context can compromise the crypto service for example if the operation context has a pointer (depending on the compiler and the platform, the pointer assignment may or may not be atomic). This violates the functional correctness requirement of the crypto service. (Concurrent calls to operations is undefined behaviour, but still should not compromise the CIA of the crypto service.)
+If, in future, we want to protect against this within the library then operations will require a status field protected by a global mutex. On entry, API calls would check the state and return an error if the state is ACTIVE. If the state is INACTIVE, then the call will set the state to ACTIVE, do the operation section and then restore the state to INACTIVE before returning.
 
-If we want to protect against this in the library, operations will need a status field protected by a global mutex similarly to key slots. On entry, API calls would check the state and return an error if it is already ACTIVE. Otherwise they set it to ACTIVE and restore it to INACTIVE before returning.
+### Future driver work
 
-Alternatively, protecting operation contexts can be left as the responsibility of the crypto service. The [PSA Crypto API Specification](https://arm-software.github.io/psa-api/crypto/1.1/overview/conventions.html#concurrent-calls) does not require the library to provide any protection in this case. A crypto service can easily add its own mutex in its operation structure wrapper (the same structure where it keeps track of which client connection owns that operation object).
+A future policy we may wish to enforce for drivers is:
 
-#### Drivers
-
-Each driver that hasn’t got the "thread_safe” property set has a dedicated mutex.
-
-Implementing "thread_safe” drivers depends on the condition variable protection in the key store, as we must guarantee that the core never starts the destruction of a key while there are operations in progress on it.
-
-Start with implementing threading for drivers without the "thread_safe” property (all drivers behave like the property wasn't set). Add "thread_safe" drivers at some point after the [Condition variables](#condition-variables) approach is implemented in the core.
-
-##### Reentrancy
-
-It is natural sometimes to want to perform cryptographic operations from a driver, for example calculating a hash as part of various other crypto primitives, or using a block cipher in a driver for a mode, etc. Also encrypting/authenticating communication with a secure element.
-
-**Non-thread-safe drivers:**
-
-A driver is non-thread-safe if the `thread-safe` property (see [Driver requirements](#driver-requirements)) is set to false.
+* By default, each driver only has at most one entry point active at any given time. In other words, each driver has its own exclusive lock.
+* Drivers have an optional `"thread_safe"` boolean property. If true, it allows concurrent calls to this driver.
+* Even with a thread-safe driver, the core never starts the destruction of a key while there are operations in progress on it, and never performs concurrent calls on the same multipart operation.
 
 In the non-thread-safe case we have these natural assumptions/requirements:
-1. Drivers don't call the core for any operation for which they provide an entry point
-2. The core doesn't hold the driver mutex between calls to entry points
 
-With these, the only way of a deadlock is when we have several drivers and they have circular dependencies. That is, Driver A makes a call that is despatched to Driver B and upon executing that Driver B makes a call that is despatched to Driver A. For example Driver A does CCM calls Driver B to do CBC-MAC, which in turn calls Driver A to do AES. This example is pretty contrived and it is hard to find a more practical example.
+1. Drivers don't call the core for any operation for which they provide an entry point.
+2. The core doesn't hold the driver mutex between calls to entry points.
+
+With these, the only way of a deadlock is when there are several drivers with circular dependencies. That is, Driver A makes a call that is dispatched to Driver B; upon executing this call Driver B makes a call that is dispatched to Driver A. For example Driver A does CCM, which calls driver B to do CBC-MAC, which in turn calls Driver A to perform AES.
 
 Potential ways for resolving this:
-1. Non-thread-safe drivers must not call the core
-2. Provide a new public API that drivers can safely call
-3. Make the dispatch layer public for drivers to call
+
+1. Non-thread-safe drivers must not call the core.
+2. Provide a new public API that drivers can safely call.
+3. Make the dispatch layer public for drivers to call.
 4. There is a whitelist of core APIs that drivers can call. Drivers providing entry points to these must not make a call to the core when handling these calls. (Drivers are still allowed to call any core API that can't have a driver entry point.)
 
 The first is too restrictive, the second and the third would require making it a stable API, and would likely increase the code size for a relatively rare feature. We are choosing the fourth as that is the most viable option.
 
 **Thread-safe drivers:**
 
-A driver is non-thread-safe if the `thread-safe` property (see [Driver requirements](#driver-requirements)) is set to true.
+A driver would be non-thread-safe if the `thread-safe` property is set to true.
 
-To make reentrancy in non-thread-safe drivers work, thread-safe drivers must not make a call to the core when handling a call that is on the non-thread-safe driver core API whitelist.
+To make re-entrancy in non-thread-safe drivers work, thread-safe drivers must not make a call to the core when handling a call that is on the non-thread-safe driver core API whitelist.
 
-Thread-safe drivers have less guarantees from the core and need to implement more complex logic and we can reasonably expect them to be more flexible in terms of reentrancy as well. At this point it is hard to see what further guarantees would be useful and feasible. Therefore, we don't provide any further guarantees for now.
+Thread-safe drivers have fewer guarantees from the core and need to implement more complex logic. We can reasonably expect them to be more flexible in terms of re-entrancy as well. At this point it is hard to see what further guarantees would be useful and feasible. Therefore, we don't provide any further guarantees for now.
 
-Thread-safe drivers must not make any assumption about the operation of the core beyond what is discussed in the [Reentrancy](#reentrancy) and [Driver requirements](#driver-requirements) sections.
-
-#### Global data
-
-PSA Crypto makes use of a `global_data` variable that will be accessible from multiple threads and needs to be protected. Any function accessing this variable (or its members) must take the corresponding lock first. Since `global_data` holds the RNG state, these will involve relatively expensive operations and therefore ideally `global_data` should be protected by its own, dedicated lock (different from the one protecting the key store).
-
-Note that this does not protect access to the RNG via `mbedtls_psa_get_random`, which is guaranteed to be thread-safe when `MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG` is disabled. Still, doing so is conceptually simpler and we probably will want to remove the lower level mutex in the long run, since the corresponding interface will be removed from the public API. The two mutexes are different and are always taken in the same order, there is no risk of deadlock.
-
-The purpose of `MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG` is very similar to the driver interface (and might even be added to it in the long run), therefore it makes sense to handle it the same way. In particular, we can use the `global_data` mutex to protect it as a default and when we implement the "thread_safe” property for drivers, we implement it for `MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG` as well.
-
-#### Implementation notes
-
-Since we only have simple mutexes, locking the same mutex from the same thread is a deadlock. Therefore functions taking the global mutex must not be called while holding the same mutex. Functions taking the mutex will document this fact and the implications.
-
-Releasing the mutex before a function call might introduce race conditions. Therefore might not be practical to take the mutex in low level access functions. If functions like that don't take the mutex, they need to rely on the caller to take it for them. These functions will document that the caller is required to hold the mutex.
-
-To avoid performance degradation, functions must hold mutexes for as short time as possible. In particular, they must not start expensive operations (eg. doing cryptography) while holding the mutex.
-
-## Strategy for 3.6
-
-The goal is to provide viable threading support without extending the platform abstraction. (Condition variables should be added in 4.0.) This means that we will be relying on mutexes only.
-
-- Key Store
-    - Slot states are described in the [Slot states](#slot-states) section. They guarantee safe concurrent access to slot contents.
-    - Slot states will be protected by a global mutex as described in the introduction of the [Global lock excluding slot content](#global-lock-excluding-slot-content) section.
-    - Simple key destruction strategy as described in the [Mutex only](#mutex-only) section (variant 2).
-    - The slot state and key attributes will be separated as described in the last paragraph of the [Determining whether a key slot is occupied](#determining-whether-a-key-slot-is-occupied) section.
-- The main `global_data` (the one in `psa_crypto.c`) shall be protected by its own mutex as described in the [Global data](#global-data) section.
-- The solution shall use the pre-existing `MBEDTLS_THREADING_C` threading abstraction. That is, the flag proposed in the [Platform abstraction](#platform-abstraction) section won't be implemented.
-- The core makes no additional guarantees for drivers. That is, Policy 1 in section [Driver requirements](#driver-requirements) applies.
+Thread-safe drivers must not make any assumption about the operation of the core beyond what is discussed here.
diff --git a/include/psa/crypto.h b/include/psa/crypto.h
index 73889e0..7083bd9 100644
--- a/include/psa/crypto.h
+++ b/include/psa/crypto.h
@@ -527,6 +527,11 @@
  * If a key is currently in use in a multipart operation, then destroying the
  * key will cause the multipart operation to fail.
  *
+ * \warning    We can only guarantee that the the key material will
+ *             eventually be wiped from memory. With threading enabled
+ *             and during concurrent execution, copies of the key material may
+ *             still exist until all threads have finished using the key.
+ *
  * \param key  Identifier of the key to erase. If this is \c 0, do nothing and
  *             return #PSA_SUCCESS.
  *