[sanitizer] Pair atomic acquire with release in BlockingMutex::Unlock

[alekseyshl]

Summary

Dmitry, seeking your expertise. I believe, the proper way to implement
Lock/Unlock here would be to use acquire/release semantics. Am I missing
something?

compiler-rt/trunk/lib/sanitizer_common/sanitizer_linux.cc

 #else
     internal_syscall(SYSCALL(futex), (uptr)m, FUTEX_WAIT, MtxSleeping, 0, 0, 0);
 #endif
   }
 }
 
 void BlockingMutex::Unlock() {
   atomic_uint32_t *m = reinterpret_cast<atomic_uint32_t *>(&opaque_storage_);
-  u32 v = atomic_exchange(m, MtxUnlocked, memory_order_relaxed);
+  u32 v = atomic_exchange(m, MtxUnlocked, memory_order_release);
   CHECK_NE(v, MtxUnlocked);
   if (v == MtxSleeping) {
 #if SANITIZER_FREEBSD
     _umtx_op(m, UMTX_OP_WAKE, 1, 0, 0);
 #else
     internal_syscall(SYSCALL(futex), (uptr)m, FUTEX_WAKE, 1, 0, 0, 0);
 #endif
   }