tsan: don't create sync objects on acquire-load
Don't create sync object if it does not exist yet. For example, an atomic
pointer is initialized to nullptr and then periodically acquire-loaded.
@@ -220,8 +220,7 @@ static a128 NoTsanAtomicLoad(const volatile a128 *a, morder mo) {
#endif
template<typename T>
-static T AtomicLoad(ThreadState *thr, uptr pc, const volatile T *a,
- morder mo) {
+static T AtomicLoad(ThreadState *thr, uptr pc, const volatile T *a, morder mo) {
CHECK(IsLoadOrder(mo));
// This fast-path is critical for performance.
// Assume the access is atomic.
@@ -229,10 +228,17 @@ static T AtomicLoad(ThreadState *thr, uptr pc, const volatile T *a,
MemoryReadAtomic(thr, pc, (uptr)a, SizeLog<T>());
return NoTsanAtomicLoad(a, mo);
}
- SyncVar *s = ctx->metamap.GetOrCreateAndLock(thr, pc, (uptr)a, false);
- AcquireImpl(thr, pc, &s->clock);
+ // Don't create sync object if it does not exist yet. For example, an atomic
+ // pointer is initialized to nullptr and then periodically acquire-loaded.
T v = NoTsanAtomicLoad(a, mo);
+ SyncVar *s = ctx->metamap.GetIfExistsAndLock((uptr)a, false);
+ if (s) {
+ AcquireImpl(thr, pc, &s->clock);
+ // Re-read under sync mutex because we need a consistent snapshot
+ // of the value and the clock we acquire.
+ v = NoTsanAtomicLoad(a, mo);
s->mtx.ReadUnlock();
+ }
MemoryReadAtomic(thr, pc, (uptr)a, SizeLog<T>());
return v;
}
