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fix race condition in file locking

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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.
This commit is contained in:
Kaarle Ritvanen 2018-09-18 10:03:27 +03:00 committed by Rich Felker
parent 1f6cbdb434
commit 0db393d3a7
1 changed files with 6 additions and 6 deletions
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12
src/stdio/__lockfile.c
View File

@ -8,13 +8,13 @@ int __lockfile(FILE *f)
int owner = f->lock, tid = __pthread_self()->tid;
if ((owner & ~MAYBE_WAITERS) == tid)
return 0;
for (;;) {
owner = a_cas(&f->lock, 0, tid);
if (!owner) return 1;
if (a_cas(&f->lock, owner, owner|MAYBE_WAITERS)==owner) break;
owner = a_cas(&f->lock, 0, tid);
if (!owner) return 1;
while ((owner = a_cas(&f->lock, 0, tid|MAYBE_WAITERS))) {
if ((owner & MAYBE_WAITERS) ||
a_cas(&f->lock, owner, owner|MAYBE_WAITERS)==owner)
__futexwait(&f->lock, owner|MAYBE_WAITERS, 1);
}
while ((owner = a_cas(&f->lock, 0, tid|MAYBE_WAITERS)))
__futexwait(&f->lock, owner, 1);
return 1;
}