From 0db393d3a77bb9f300a356c6a5484fc2dddb161d Mon Sep 17 00:00:00 2001 From: Kaarle Ritvanen Date: Tue, 18 Sep 2018 10:03:27 +0300 Subject: [PATCH] fix race condition in file locking 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 c21f750727515602a9e84f2a190ee8a0a2aeb2a1. Signed-off-by: Thomas Petazzoni --- src/stdio/__lockfile.c | 12 ++++++------ 1 file changed, 6 insertions(+), 6 deletions(-) diff --git a/src/stdio/__lockfile.c b/src/stdio/__lockfile.c index 2ff75d8a..0dcb2a42 100644 --- a/src/stdio/__lockfile.c +++ b/src/stdio/__lockfile.c @@ -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; } -- 2.14.4