From: Christoph Hellwig <hch@infradead.org>
To: Ben Myers <bpm@sgi.com>
Cc: Dave Chinner <dchinner@redhat.com>, xfs@oss.sgi.com
Subject: Re: [PATCH 4/4] xfs: fix the logspace waiting algorithm
Date: Fri, 2 Dec 2011 06:51:41 -0500 [thread overview]
Message-ID: <20111202115141.GA21643@infradead.org> (raw)
In-Reply-To: <20111201195128.GZ29840@sgi.com>
On Thu, Dec 01, 2011 at 01:51:28PM -0600, Ben Myers wrote:
> Process A reads from the grant reserve head at 2641 (and there currently is enough space)
> Process B wakes at either 2646 or 2650, in xlog_reserveq_wait, locks, and reads from the grant reserve head (and currently there is enough space)
> Process B removes itself from the list
> Process A reads from the reservq list and finds it to be empty
> Process A finds that there was enough space at 2646
> Process B returns from xlog_reserveq_wait, unlocks, grants space at 2656, returns
> Process A grants log space at 2656, and returns
>
> AFAICS there is nothing that prevents these guys from granting the same
> space when you approach free_bytes >= need_bytes concurrently.
>
> This lockless stuff is always a mind job for me. I'll take another look at
> some of the other aspects of the patch. Even if it doesn't resolve my
> question about the lockless issue, it seems to resolve Chandra's race.
Indeed, I think we have this race. Then again I I think we had
exactly the same one before, too. The only way to fix it would be
to do a sort of double cmpxchg that only moves the grant head forward
if it's still in available space vs the tails lsn.
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next prev parent reply other threads:[~2011-12-02 11:52 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-11-28 8:17 [PATCH 0/4] xfs fixes for Linux 3.2-rc3 Christoph Hellwig
2011-11-28 8:17 ` [PATCH 1/4] xfs: fix attr2 vs large data fork assert Christoph Hellwig
2011-11-28 18:01 ` Ben Myers
2011-11-28 18:03 ` Christoph Hellwig
2011-11-28 8:17 ` [PATCH 2/4] xfs: validate acl count Christoph Hellwig
2011-11-28 8:17 ` [PATCH 3/4] xfs: force buffer writeback before blocking on the ilock in inode reclaim Christoph Hellwig
2011-11-28 8:17 ` [PATCH 4/4] xfs: fix the logspace waiting algorithm Christoph Hellwig
2011-11-30 23:56 ` Ben Myers
2011-12-01 7:21 ` Christoph Hellwig
2011-12-01 19:51 ` Ben Myers
2011-12-02 11:51 ` Christoph Hellwig [this message]
2011-12-05 2:53 ` Dave Chinner
2011-12-05 9:05 ` Christoph Hellwig
2011-12-01 18:32 ` Ben Myers
2011-12-01 20:43 ` Chandra Seetharaman
2011-12-01 19:28 ` Chandra Seetharaman
2011-12-02 11:16 ` Christoph Hellwig
2011-12-02 16:02 ` Chandra Seetharaman
2011-11-29 19:23 ` [PATCH 0/4] xfs fixes for Linux 3.2-rc3 Ben Myers
2011-11-30 8:54 ` Christoph Hellwig
2011-11-30 8:58 ` [PATCH 5/4] xfs: fix nfs export of 64-bit inodes numbers on 32-bit kernels Christoph Hellwig
2011-12-02 16:07 ` Ben Myers
2011-12-02 17:29 ` Christoph Hellwig
2011-12-06 16:39 ` Ben Myers
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