diff options
Diffstat (limited to 'absl/base/call_once.h')
| -rw-r--r-- | absl/base/call_once.h | 13 |
1 files changed, 11 insertions, 2 deletions
diff --git a/absl/base/call_once.h b/absl/base/call_once.h index 4aa6360..e1614e5 100644 --- a/absl/base/call_once.h +++ b/absl/base/call_once.h @@ -148,7 +148,7 @@ void CallOnceImpl(std::atomic<uint32_t>* control, Args&&... args) { #ifndef NDEBUG { - uint32_t old_control = control->load(std::memory_order_acquire); + uint32_t old_control = control->load(std::memory_order_relaxed); if (old_control != kOnceInit && old_control != kOnceRunning && old_control != kOnceWaiter && @@ -166,14 +166,23 @@ void CallOnceImpl(std::atomic<uint32_t>* control, // Must do this before potentially modifying control word's state. base_internal::SchedulingHelper maybe_disable_scheduling(scheduling_mode); // Short circuit the simplest case to avoid procedure call overhead. + // The base_internal::SpinLockWait() call returns either kOnceInit or + // kOnceDone. If it returns kOnceDone, it must have loaded the control word + // with std::memory_order_acquire and seen a value of kOnceDone. uint32_t old_control = kOnceInit; if (control->compare_exchange_strong(old_control, kOnceRunning, - std::memory_order_acquire, std::memory_order_relaxed) || base_internal::SpinLockWait(control, ABSL_ARRAYSIZE(trans), trans, scheduling_mode) == kOnceInit) { base_internal::Invoke(std::forward<Callable>(fn), std::forward<Args>(args)...); + // The call to SpinLockWake below is an optimization, because the waiter + // in SpinLockWait is waiting with a short timeout. The atomic load/store + // sequence is slightly faster than an atomic exchange: + // old_control = control->exchange(base_internal::kOnceDone, + // std::memory_order_release); + // We opt for a slightly faster case when there are no waiters, in spite + // of longer tail latency when there are waiters. old_control = control->load(std::memory_order_relaxed); control->store(base_internal::kOnceDone, std::memory_order_release); if (old_control == base_internal::kOnceWaiter) { |