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Date: Tue, 18 Sep 2018 11:55:08 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: [PATCH v2] fix race condition in file locking
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
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