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Date: Mon, 17 Sep 2018 19:19:07 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] fix race condition in file locking
On Tue, Sep 18, 2018 at 01:53:36AM +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 | 11 +++++------
> 1 file changed, 5 insertions(+), 6 deletions(-)
>
> diff --git a/src/stdio/__lockfile.c b/src/stdio/__lockfile.c
> index 2ff75d8a..f6adefb6 100644
> --- a/src/stdio/__lockfile.c
> +++ b/src/stdio/__lockfile.c
> @@ -8,13 +8,12 @@ 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 (a_cas(&f->lock, owner, owner|MAYBE_WAITERS)==owner)
> + __futexwait(&f->lock, owner, 1);
> }
Overall I think the concept is right, but I don't understand the logic
in this loop. When adding the MAYBE_WAITERS flag with a_cas succeeds,
it will try to __futexwait with the old value that doesn't have
MAYBE_WAITERS set, which will fail and spin again, adding a spurious
cas and a spurious syscall.
I think the right logic would be something like:
while ((owner = a_cas(&f->lock, 0, tid|MAYBE_WAITERS))) {
if (!(owner & MAYBE_WAITERS))
a_cas(&f->lock, owner, owner|MAYBE_WAITERS);
__futexwait(&f->lock, owner|MAYBE_WAITERS, 1);
}
Does that look better to you? The if is not actually necessary (the
a_cas could be done unconditionally) but skipping it puts less
pressure on the hardware's cache coherency.
Alternatively it could be:
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);
}
This form does what you were trying to do, I think, and avoids making
the syscall when the owner changed in a race (the syscall would fail,
but avoiding it saves time).
Rich
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