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[*] Harden WinXP-7 semaphores [2b0ed797 cont] [more paranoia]

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[*] Dont overspin spurious modernnt condvar wakes
[*] Revert 2943ffdb's add waiter, if under modern nt

1: While spuriously waking a blocked keyed event, don't leave the condition variable in a state where signals can be ignored.
This commit is contained in:
Reece Wilson 2024-02-10 15:59:35 +00:00
parent c1ccb81b7b
commit c111dee855
1 changed files with 16 additions and 15 deletions
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31
Source/Threading/Primitives/AuConditionVariable.NT.cpp
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@ -151,8 +151,9 @@ namespace Aurora::Threading::Primitives
}
else
{
if (bIOU ||
(bSpin ? this->CheckOut() : this->CheckOutNoSpin()))
// Acquire our signal before spinning and potentially giving up on our timeslice.
// If we were awoken, try to acquire before yielding to a contested mutex.
if (bIOU || this->CheckOutNoSpin())
{
pMutex->Lock();
return true;
@ -160,10 +161,11 @@ namespace Aurora::Threading::Primitives
if (!gUseNativeWaitCondvar)
{
// (NT 5-6.1) Unblocks one race condition, where another thread checks out our signal, and we return with nothing.
// Normal execution of the blocked thread can continue, so long as we trigger the keyed event as though this thread was signaled.
// This is only relevant in an edge case of multi-waiters spinning, rapid signaling, and the IOU being hit once. Do this instead of ticketing.
this->SignalSpuriously();
}
this->AddWaiter();
}
}
}
@ -211,8 +213,7 @@ namespace Aurora::Threading::Primitives
pNtWaitForKeyedEvent(gKeyedEventHandle, (void *)&this->wlist, 0, nullptr);
}
if (bIOU ||
(bSpin ? this->CheckOut() : this->CheckOutNoSpin()))
if (bIOU || this->CheckOutNoSpin())
{
pMutex->Lock();
return bRet;
@ -222,8 +223,6 @@ namespace Aurora::Threading::Primitives
{
this->SignalSpuriously();
}
this->AddWaiter();
}
}
#else
@ -344,20 +343,22 @@ namespace Aurora::Threading::Primitives
if (expected)
{
// NOTE: missing this->signalCount atomic increment to force another AddWaiter under successful keyedevent return
// INTENTIONAL: Missing this->signalCount atomic increment to force another AddWaiter under successful keyedevent return
while (expected)
{
if (AuAtomicCompareExchange(&this->wlist, ((expected - 1) << kShiftCountByBits) /*intentional clear*/, original) == original)
// INTENTIONAL: Removed the -1 so we dont have to rewatch the condvar, potentially missing a signal in the process
if (AuAtomicCompareExchange(&this->wlist, (expected << kShiftCountByBits) /*intentional clear*/, original) == original)
{
if (gUseNativeWaitCondvar)
{
InternalLTSWakeOne((void *)&this->wlist);
}
else
if (!gUseNativeWaitCondvar)
{
pNtReleaseKeyedEvent(gKeyedEventHandle, (void *)&this->wlist, FALSE, nullptr);
}
// INTENTIONAL: Removal of modernt branch. We err on the side of caution by failing awake, then its up to the last waking
// thread up process to block again. So long as wlist represents all the waiting threads, it doesn't matter,
// we will continue to be signalable withouts signal being lost or any thread blocking.
return;
}