* [PATCH v2] fix race condition in file locking
@ 2018-09-18 7:03 Kaarle Ritvanen
2018-09-18 15:55 ` Rich Felker
0 siblings, 1 reply; 2+ messages in thread
From: Kaarle Ritvanen @ 2018-09-18 7:03 UTC (permalink / raw)
To: musl; +Cc: Kaarle Ritvanen
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.
---
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
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: [PATCH v2] fix race condition in file locking
2018-09-18 7:03 [PATCH v2] fix race condition in file locking Kaarle Ritvanen
@ 2018-09-18 15:55 ` Rich Felker
0 siblings, 0 replies; 2+ messages in thread
From: Rich Felker @ 2018-09-18 15:55 UTC (permalink / raw)
To: musl
On Tue, Sep 18, 2018 at 10:03:27AM +0300, Kaarle Ritvanen wrote:
> 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.
> ---
> 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
Thanks! I'm applying this with a note added to the commit message
about the source of the regression. I hit the same race today working
on a test case for something else, so I can confirm it occurs in
practice. Distros using musl 1.1.20 should apply this patch.
Rich
^ permalink raw reply [flat|nested] 2+ messages in thread
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2018-09-18 15:55 ` Rich Felker
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