kernel/include/kswap.h

/*
 * Copyright (c) 2018 Intel Corporation
 *
 * SPDX-License-Identifier: Apache-2.0
 */
#ifndef ZEPHYR_KERNEL_INCLUDE_KSWAP_H_
#define ZEPHYR_KERNEL_INCLUDE_KSWAP_H_

#include <ksched.h>
#include <spinlock.h>
#include <kernel_arch_func.h>

#ifdef CONFIG_STACK_SENTINEL
extern void z_check_stack_sentinel(void);
#else
#define z_check_stack_sentinel() /**/
#endif

/* In SMP, the irq_lock() is a spinlock which is implicitly released
 * and reacquired on context switch to preserve the existing
 * semantics.  This means that whenever we are about to return to a
 * thread (via either z_swap() or interrupt/exception return!) we need
 * to restore the lock state to whatever the thread's counter
 * expects.
 */
void z_smp_reacquire_global_lock(struct k_thread *thread);
void z_smp_release_global_lock(struct k_thread *thread);

/* context switching and scheduling-related routines */
#ifdef CONFIG_USE_SWITCH

/* There is an unavoidable SMP race when threads swap -- their thread
 * record is in the queue (and visible to other CPUs) before
 * arch_switch() finishes saving state.  We must spin for the switch
 * handle before entering a new thread.  See docs on arch_switch().
 *
 * Note: future SMP architectures may need a fence/barrier or cache
 * invalidation here.  Current ones don't, and sadly Zephyr doesn't
 * have a framework for that yet.
 */
static inline void wait_for_switch(struct k_thread *thread)
{
#ifdef CONFIG_SMP
	volatile void **shp = (void *)&thread->switch_handle;

	while (*shp == NULL) {
		k_busy_wait(1);
	}
#endif
}

/* New style context switching.  arch_switch() is a lower level
 * primitive that doesn't know about the scheduler or return value.
 * Needed for SMP, where the scheduler requires spinlocking that we
 * don't want to have to do in per-architecture assembly.
 *
 * Note that is_spinlock is a compile-time construct which will be
 * optimized out when this function is expanded.
 */
static ALWAYS_INLINE unsigned int do_swap(unsigned int key,
					  struct k_spinlock *lock,
					  int is_spinlock)
{
	ARG_UNUSED(lock);
	struct k_thread *new_thread, *old_thread;

	old_thread = _current;

	z_check_stack_sentinel();

	if (is_spinlock) {
		k_spin_release(lock);
	}

#ifdef CONFIG_SMP
	/* Null out the switch handle, see wait_for_switch() above.
	 * Note that we set it back to a non-null value if we are not
	 * switching!  The value itself doesn't matter, because by
	 * definition _current is running and has no saved state.
	 */
	volatile void **shp = (void *)&old_thread->switch_handle;

	*shp = NULL;
#endif

	new_thread = z_get_next_ready_thread();

#ifdef CONFIG_SMP
	if (new_thread == old_thread) {
		*shp = old_thread;
	}
#endif

	if (new_thread != old_thread) {
#ifdef CONFIG_TIMESLICING
		z_reset_time_slice();
#endif

		old_thread->swap_retval = -EAGAIN;

#ifdef CONFIG_SMP
		_current_cpu->swap_ok = 0;
		new_thread->base.cpu = arch_curr_cpu()->id;

		if (!is_spinlock) {
			z_smp_release_global_lock(new_thread);
		}
#endif
		z_thread_mark_switched_out();
		wait_for_switch(new_thread);
		arch_cohere_stacks(old_thread, NULL, new_thread);
		_current_cpu->current = new_thread;

		arch_switch(new_thread->switch_handle,
			     &old_thread->switch_handle);
	}

	if (is_spinlock) {
		arch_irq_unlock(key);
	} else {
		irq_unlock(key);
	}

	return _current->swap_retval;
}

static inline int z_swap_irqlock(unsigned int key)
{
	return do_swap(key, NULL, 0);
}

static inline int z_swap(struct k_spinlock *lock, k_spinlock_key_t key)
{
	return do_swap(key.key, lock, 1);
}

static inline void z_swap_unlocked(void)
{
	struct k_spinlock lock = {};
	k_spinlock_key_t key = k_spin_lock(&lock);

	(void) z_swap(&lock, key);
}

#else /* !CONFIG_USE_SWITCH */

extern int arch_swap(unsigned int key);

static inline int z_swap_irqlock(unsigned int key)
{
	int ret;
	z_check_stack_sentinel();
	ret = arch_swap(key);
	return ret;
}

/* If !USE_SWITCH, then spinlocks are guaranteed degenerate as we
 * can't be in SMP.  The k_spin_release() call is just for validation
 * handling.
 */
static ALWAYS_INLINE int z_swap(struct k_spinlock *lock, k_spinlock_key_t key)
{
	k_spin_release(lock);
	return z_swap_irqlock(key.key);
}

static inline void z_swap_unlocked(void)
{
	(void) z_swap_irqlock(arch_irq_lock());
}

#endif /* !CONFIG_USE_SWITCH */

/**
 * Set up a "dummy" thread, used at early initialization to launch the
 * first thread on a CPU.
 *
 * Needs to set enough fields such that the context switching code can
 * use it to properly store state, which will just be discarded.
 *
 * The memory of the dummy thread can be completely uninitialized.
 */
static inline void z_dummy_thread_init(struct k_thread *dummy_thread)
{
	dummy_thread->base.thread_state = _THREAD_DUMMY;
#ifdef CONFIG_SCHED_CPU_MASK
	dummy_thread->base.cpu_mask = -1;
#endif
	dummy_thread->base.user_options = K_ESSENTIAL;
#ifdef CONFIG_THREAD_STACK_INFO
	dummy_thread->stack_info.start = 0U;
	dummy_thread->stack_info.size = 0U;
#endif
#ifdef CONFIG_USERSPACE
	dummy_thread->mem_domain_info.mem_domain = &k_mem_domain_default;
#endif

	_current_cpu->current = dummy_thread;
}
#endif /* ZEPHYR_KERNEL_INCLUDE_KSWAP_H_ */