| From 0db393d3a77bb9f300a356c6a5484fc2dddb161d Mon Sep 17 00:00:00 2001 |
| From: Kaarle Ritvanen <kaarle.ritvanen@datakunkku.fi> |
| 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 <thomas.petazzoni@bootlin.com> |
| --- |
| 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 |
| |