Move `absl::MutexLock sample_lock` out of the global graveyard critical section.

PiperOrigin-RevId: 985732017
Change-Id: I3e31f60efa867da97e0d833b7dfc3e0a6870c801

absl/profiling/internal/sample_recorder.h

diff --git a/absl/profiling/internal/sample_recorder.h b/absl/profiling/internal/sample_recorder.h
index 8663d6c..0d14027 100644
--- a/absl/profiling/internal/sample_recorder.h
+++ b/absl/profiling/internal/sample_recorder.h
@@ -169,29 +169,23 @@
 template <typename T>
 template <typename... Targs>
 T* SampleRecorder<T>::PopDead(Targs... args) {
-  T* sample;
-  {
-    absl::MutexLock graveyard_lock(graveyard_.init_mu);
+  absl::ReleasableMutexLock graveyard_lock(graveyard_.init_mu);
 
-    // The list is circular, so eventually it collapses down to
-    //   graveyard_.dead == &graveyard_
-    // when it is empty.
-    sample = graveyard_.dead;
-    if (sample == &graveyard_) return nullptr;
+  // The list is circular, so eventually it collapses down to
+  //   graveyard_.dead == &graveyard_
+  // when it is empty.
+  T* sample = graveyard_.dead;
+  if (sample == &graveyard_) return nullptr;
 
-    graveyard_.dead = ABSL_TS_UNCHECKED_READ(sample->dead);
-    // Release the global graveyard lock early, before the potentially slow
-    // preparation.
-  }
-  // `sample` is detached from the graveyard and will not be used in PopDead.
-  // SampleRecorder<T>::Iterate will acquire per sample lock, so there will be
-  // no data race either.
   absl::MutexLock sample_lock(sample->init_mu);
-  // Note: consider PrepareForSampling out of the lock.
-  // Currently many clients have PrepareForSampling marked as
-  // ABSL_EXCLUSIVE_LOCKS_REQUIRED.
-  sample->PrepareForSampling(std::forward<Targs>(args)...);
+  graveyard_.dead = sample->dead;
+  // Release the global graveyard lock early, before the potentially slow
+  // preparation.
+  graveyard_lock.Release();
+  // Prepare the sample while still holding the per-sample lock.
+  // `Iterate` will wait for the lock to be released.
   sample->dead = nullptr;
+  sample->PrepareForSampling(std::forward<Targs>(args)...);
   return sample;
 }