Bill Tutt:

Calling Sleep(0) for a spinlock can cause a priority inversion, adding
comments to explain what's going on.

diff --git a/Python/thread_nt.h b/Python/thread_nt.h
index 0c7ced4abbe1206ab7cd3b4da59053a914a850f8..c8ed6cd70e642bc1a2a5f5fc1a2aa567f565a34d 100644 (file)
--- a/Python/thread_nt.h
+++ b/Python/thread_nt.h
@@ -50,9 +50,32 @@ static PVOID WINAPI interlocked_cmp_xchg(PVOID *dest, PVOID exc, PVOID comperand
 {
        static LONG spinlock = 0 ;
        PVOID result ;
+       DWORD dwSleep = 0;
 
        /* Acqire spinlock (yielding control to other threads if cant aquire for the moment) */
-       while(InterlockedExchange(&spinlock, 1)) Sleep(0) ;
+       while(InterlockedExchange(&spinlock, 1))
+       {
+               // Using Sleep(0) can cause a priority inversion.
+               // Sleep(0) only yields the processor if there's
+               // another thread of the same priority that's
+               // ready to run.  If a high-priority thread is
+               // trying to acquire the lock, which is held by
+               // a low-priority thread, then the low-priority
+               // thread may never get scheduled and hence never
+               // free the lock.  NT attempts to avoid priority
+               // inversions by temporarily boosting the priority
+               // of low-priority runnable threads, but the problem
+               // can still occur if there's a medium-priority
+               // thread that's always runnable.  If Sleep(1) is used,
+               // then the thread unconditionally yields the CPU.  We
+               // only do this for the second and subsequent even
+               // iterations, since a millisecond is a long time to wait
+               // if the thread can be scheduled in again sooner
+               // (~100,000 instructions).
+               // Avoid priority inversion: 0, 1, 0, 1,...
+               Sleep(dwSleep);
+               dwSleep = !dwSleep;
+       }
        result = *dest ;
        if (result == comperand)
                *dest = exc ;