Kaarle RitvanenandRich Felker
0db393d3a7
The condition occurs when
- thread #1 is holding the lock
- thread #2 is waiting for it on __futexwait
- thread #1 is about to release the lock and performs a_swap
- thread #3 enters the __lockfile function and manages to grab the lock
before thread #1 calls __wake, resetting the MAYBE_WAITERS flag
- thread #1 calls __wake
- thread #2 wakes up but goes again to __futexwait as the lock is
held by thread #3
- thread #3 releases the lock but does not call __wake as the
MAYBE_WAITERS flag is not set
This condition results in thread #2 not being woken up. This patch fixes
the problem by making the woken up thread ensure that the flag is
properly set before going to sleep again.
Mainainer's note: This fixes a regression introduced in commit
c21f750727.
26 lines
585 B
C
26 lines
585 B
C
#include "stdio_impl.h"
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#include "pthread_impl.h"
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#define MAYBE_WAITERS 0x40000000
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int __lockfile(FILE *f)
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{
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int owner = f->lock, tid = __pthread_self()->tid;
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if ((owner & ~MAYBE_WAITERS) == tid)
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return 0;
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owner = a_cas(&f->lock, 0, tid);
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if (!owner) return 1;
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while ((owner = a_cas(&f->lock, 0, tid|MAYBE_WAITERS))) {
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if ((owner & MAYBE_WAITERS) ||
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a_cas(&f->lock, owner, owner|MAYBE_WAITERS)==owner)
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__futexwait(&f->lock, owner|MAYBE_WAITERS, 1);
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}
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return 1;
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}
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void __unlockfile(FILE *f)
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{
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if (a_swap(&f->lock, 0) & MAYBE_WAITERS)
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__wake(&f->lock, 1, 1);
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}
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