mirror of https://github.com/python/cpython
Bill Tutt:
Calling Sleep(0) for a spinlock can cause a priority inversion, adding comments to explain what's going on.
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@ -50,9 +50,32 @@ static PVOID WINAPI interlocked_cmp_xchg(PVOID *dest, PVOID exc, PVOID comperand
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{
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static LONG spinlock = 0 ;
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PVOID result ;
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DWORD dwSleep = 0;
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/* Acqire spinlock (yielding control to other threads if cant aquire for the moment) */
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while(InterlockedExchange(&spinlock, 1)) Sleep(0) ;
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while(InterlockedExchange(&spinlock, 1))
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{
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// Using Sleep(0) can cause a priority inversion.
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// Sleep(0) only yields the processor if there's
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// another thread of the same priority that's
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// ready to run. If a high-priority thread is
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// trying to acquire the lock, which is held by
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// a low-priority thread, then the low-priority
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// thread may never get scheduled and hence never
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// free the lock. NT attempts to avoid priority
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// inversions by temporarily boosting the priority
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// of low-priority runnable threads, but the problem
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// can still occur if there's a medium-priority
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// thread that's always runnable. If Sleep(1) is used,
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// then the thread unconditionally yields the CPU. We
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// only do this for the second and subsequent even
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// iterations, since a millisecond is a long time to wait
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// if the thread can be scheduled in again sooner
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// (~100,000 instructions).
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// Avoid priority inversion: 0, 1, 0, 1,...
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Sleep(dwSleep);
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dwSleep = !dwSleep;
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}
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result = *dest ;
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if (result == comperand)
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*dest = exc ;
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