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TSan: Support relaxed accesses and fences #142579

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TSan: Support relaxed accesses and fences
rymrg Jun 3, 2025
2c0ed14
fixup! TSan: Support relaxed accesses and fences
rymrg Jun 20, 2025
37a0ae5
fixup! fixup! TSan: Support relaxed accesses and fences
rymrg Jul 10, 2025
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3 changes: 3 additions & 0 deletions compiler-rt/lib/tsan/rtl/tsan_flags.inc
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Original file line number Diff line number Diff line change
Expand Up @@ -80,3 +80,6 @@ TSAN_FLAG(bool, shared_ptr_interceptor, true,
TSAN_FLAG(bool, print_full_thread_history, false,
"If set, prints thread creation stacks for the threads involved in "
"the report and their ancestors up to the main thread.")
TSAN_FLAG(bool, correct_race_detection, false,
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Nit: please give a more specific name for the flag (and associated variable) - something like "precise_relaxed_access_checks". Otherwise, if anyone makes another improvement to TSan, their flag would be "even_more_correct_race_detection".

"If set, remove optimizations and execute correct race detection "
"supporting fences and release sequence.")
104 changes: 81 additions & 23 deletions compiler-rt/lib/tsan/rtl/tsan_interface_atomic.cpp
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Expand Up @@ -27,6 +27,10 @@

using namespace __tsan;

static bool correctRaceDetection(){
return flags()->correct_race_detection;
}

#if !SANITIZER_GO && __TSAN_HAS_INT128
// Protects emulation of 128-bit atomic operations.
static StaticSpinMutex mutex128;
Expand Down Expand Up @@ -227,18 +231,36 @@ namespace {
template <typename T, T (*F)(volatile T *v, T op)>
static T AtomicRMW(ThreadState *thr, uptr pc, volatile T *a, T v, morder mo) {
MemoryAccess(thr, pc, (uptr)a, AccessSize<T>(), kAccessWrite | kAccessAtomic);
if (LIKELY(mo == mo_relaxed))
return F(a, v);
if (!correctRaceDetection()){
if (LIKELY(mo == mo_relaxed))
return F(a, v);
}
SlotLocker locker(thr);
{
auto s = ctx->metamap.GetSyncOrCreate(thr, pc, (uptr)a, false);
RWLock lock(&s->mtx, IsReleaseOrder(mo));
if (IsAcqRelOrder(mo))
thr->clock.ReleaseAcquire(&s->clock);
else if (IsReleaseOrder(mo))
thr->clock.Release(&s->clock);
else if (IsAcquireOrder(mo))
thr->clock.Acquire(s->clock);
bool fullLock = correctRaceDetection() || IsReleaseOrder(mo);
RWLock lock(&s->mtx, fullLock);
if (!correctRaceDetection()){
if (IsAcqRelOrder(mo))
thr->clock.ReleaseAcquire(&s->clock);
else if (IsReleaseOrder(mo))
thr->clock.Release(&s->clock);
else if (IsAcquireOrder(mo))
thr->clock.Acquire(s->clock);
} else {
if (mo == mo_relaxed){
thr->clockA.Acquire(s->clock);
thr->clockR.Release(&s->clock);
} else if (IsAcqRelOrder(mo)) {
thr->clock.ReleaseAcquire(&s->clock);
} else if (IsReleaseOrder(mo)) {
thr->clockA.Acquire(s->clock);
thr->clock.Release(&s->clock);
} else if (IsAcquireOrder(mo)) {
thr->clock.Acquire(s->clock);
thr->clockR.Release(&s->clock);
}
}
v = F(a, v);
}
if (IsReleaseOrder(mo))
Expand All @@ -264,7 +286,7 @@ struct OpLoad {
DCHECK(IsLoadOrder(mo));
// This fast-path is critical for performance.
// Assume the access is atomic.
if (!IsAcquireOrder(mo)) {
if (!correctRaceDetection() && !IsAcquireOrder(mo)) {
MemoryAccess(thr, pc, (uptr)a, AccessSize<T>(),
kAccessRead | kAccessAtomic);
return NoTsanAtomic(mo, a);
Expand All @@ -276,7 +298,11 @@ struct OpLoad {
if (s) {
SlotLocker locker(thr);
ReadLock lock(&s->mtx);
thr->clock.Acquire(s->clock);
if (IsAcquireOrder(mo)) {
thr->clock.Acquire(s->clock);
} else if (correctRaceDetection()) {
thr->clockA.Acquire(s->clock);
}
// Re-read under sync mutex because we need a consistent snapshot
// of the value and the clock we acquire.
v = NoTsanAtomic(mo, a);
Expand Down Expand Up @@ -309,18 +335,22 @@ struct OpStore {
// Assume the access is atomic.
// Strictly saying even relaxed store cuts off release sequence,
// so must reset the clock.
if (!IsReleaseOrder(mo)) {
if (!correctRaceDetection() && !IsReleaseOrder(mo)) {
NoTsanAtomic(mo, a, v);
return;
}
SlotLocker locker(thr);
{
auto s = ctx->metamap.GetSyncOrCreate(thr, pc, (uptr)a, false);
Lock lock(&s->mtx);
thr->clock.ReleaseStore(&s->clock);
if (IsReleaseOrder(mo))
thr->clock.ReleaseStore(&s->clock);
else if (correctRaceDetection())
thr->clockR.ReleaseStore(&s->clock);
NoTsanAtomic(mo, a, v);
}
IncrementEpoch(thr);
if (IsReleaseOrder(mo))
IncrementEpoch(thr);
}
};

Expand Down Expand Up @@ -441,7 +471,7 @@ struct OpCAS {

MemoryAccess(thr, pc, (uptr)a, AccessSize<T>(),
kAccessWrite | kAccessAtomic);
if (LIKELY(mo == mo_relaxed && fmo == mo_relaxed)) {
if (LIKELY(!correctRaceDetection() && mo == mo_relaxed && fmo == mo_relaxed)) {
T cc = *c;
T pr = func_cas(a, cc, v);
if (pr == cc)
Expand All @@ -454,20 +484,36 @@ struct OpCAS {
bool success;
{
auto s = ctx->metamap.GetSyncOrCreate(thr, pc, (uptr)a, false);
RWLock lock(&s->mtx, release);
bool fullLock = correctRaceDetection() || release;
RWLock lock(&s->mtx, fullLock);
T cc = *c;
T pr = func_cas(a, cc, v);
success = pr == cc;
if (!success) {
*c = pr;
mo = fmo;
}
if (success && IsAcqRelOrder(mo))
thr->clock.ReleaseAcquire(&s->clock);
else if (success && IsReleaseOrder(mo))
thr->clock.Release(&s->clock);
else if (IsAcquireOrder(mo))
thr->clock.Acquire(s->clock);
if (!correctRaceDetection()){
if (success && IsAcqRelOrder(mo))
thr->clock.ReleaseAcquire(&s->clock);
else if (success && IsReleaseOrder(mo))
thr->clock.Release(&s->clock);
else if (IsAcquireOrder(mo))
thr->clock.Acquire(s->clock);
} else {
if (!IsAcquireOrder(mo)){
thr->clockA.Acquire(s->clock);
} else {
thr->clock.Acquire(s->clock);
}
if (success){
if (!IsReleaseOrder(mo)){
thr->clockR.Release(&s->clock);
} else {
thr->clock.Release(&s->clock);
}
}
}
}
if (success && release)
IncrementEpoch(thr);
Expand All @@ -487,7 +533,19 @@ struct OpFence {
static void NoTsanAtomic(morder mo) { __sync_synchronize(); }

static void Atomic(ThreadState *thr, uptr pc, morder mo) {
// FIXME(dvyukov): not implemented.
if (correctRaceDetection()){
SlotLocker locker(thr);
if (IsAcquireOrder(mo))
thr->clock.Acquire(&thr->clockA);
if (mo == mo_seq_cst){
auto s = ctx->metamap.GetSyncOrCreate(thr, pc, 0, false);
thr->clock.ReleaseAcquire(&s->clock);
}
if (IsReleaseOrder(mo)){
thr->clockR.Acquire(&thr->clock);
IncrementEpoch(thr);
}
}
__sync_synchronize();
}
};
Expand Down
2 changes: 2 additions & 0 deletions compiler-rt/lib/tsan/rtl/tsan_rtl.h
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Expand Up @@ -177,6 +177,8 @@ struct alignas(SANITIZER_CACHE_LINE_SIZE) ThreadState {
atomic_sint32_t pending_signals;

VectorClock clock;
VectorClock clockR;
VectorClock clockA;

// This is a slow path flag. On fast path, fast_state.GetIgnoreBit() is read.
// We do not distinguish beteween ignoring reads and writes
Expand Down