Date: Thu, 8 May 2008 09:37:00 +0100 From: Doug Rabson <dfr@rabson.org> To: paul.koch@statseeker.com Cc: freebsd-stable@freebsd.org Subject: Re: flock incorrectly detects deadlock on 7-stable and current Message-ID: <1B6FCF23-413B-452A-B66D-3CCD6257F7BD@rabson.org> In-Reply-To: <200805081812.24692.paul.koch@statseeker.com> References: <200805081812.24692.paul.koch@statseeker.com>
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On 8 May 2008, at 09:12, Paul Koch wrote:
> Hi,
>
> We have been trying to track down a problem with one of our apps which
> does a lot of flock(2) calls. flock returns errno 11 (Resource
> deadlock avoided) under certain scenarios. Our app works fine on
> 7-Release, but fails on 7-stable and -current.
>
> The problem appears to be when we have at least three processes doing
> flock() on a file, and one is trying to upgrade a shared lock to an
> exclusive lock but fails with a deadlock avoided.
>
> Attached is a simple flock() test program.
>
> a. Process 1 requests and gets a shared lock
> b. Process 2 requests and blocks for an exclusive lock
> c. Process 3 requests and gets a shared lock
> d. Process 3 requests an upgrade to an exclusive lock but fails (errno
> 11)
>
> If we change 'd' to
> Process 3 requests unlock, then requests exclusive lock, it works.
Could you possibly try this patch and tell me if it helps:
==== //depot/user/dfr/lockd/sys/kern/kern_lockf.c#57 - /tank/projects/
lockd/src/sys/kern/kern_lockf.c ====
@@ -1370,6 +1370,18 @@
}
/*
+ * For flock type locks, we must first remove
+ * any shared locks that we hold before we sleep
+ * waiting for an exclusive lock.
+ */
+ if ((lock->lf_flags & F_FLOCK) &&
+ lock->lf_type == F_WRLCK) {
+ lock->lf_type = F_UNLCK;
+ lf_activate_lock(state, lock);
+ lock->lf_type = F_WRLCK;
+ }
+
+ /*
* We are blocked. Create edges to each blocking lock,
* checking for deadlock using the owner graph. For
* simplicity, we run deadlock detection for all
@@ -1389,17 +1401,6 @@
}
/*
- * For flock type locks, we must first remove
- * any shared locks that we hold before we sleep
- * waiting for an exclusive lock.
- */
- if ((lock->lf_flags & F_FLOCK) &&
- lock->lf_type == F_WRLCK) {
- lock->lf_type = F_UNLCK;
- lf_activate_lock(state, lock);
- lock->lf_type = F_WRLCK;
- }
- /*
* We have added edges to everything that blocks
* us. Sleep until they all go away.
*/
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