From: Christoph Hellwig <hch@infradead.org>
To: Dave Chinner <david@fromorbit.com>
Cc: Christoph Hellwig <hch@infradead.org>, xfs@oss.sgi.com
Subject: Re: [PATCH 2/5] xfs: use xfs_ilock_map_shared in xfs_qm_dqtobp
Date: Thu, 5 Dec 2013 12:53:45 -0800 [thread overview]
Message-ID: <20131205205345.GA12393@infradead.org> (raw)
In-Reply-To: <20131205204108.GB29897@dastard>
On Fri, Dec 06, 2013 at 07:41:08AM +1100, Dave Chinner wrote:
> However, it raises a bigger question about dquot allocation sanity
> to me: what makes the map returned valid after we've unlocked the
> extent list?
>
> We then use it to determine whether to allocate a
> dquot or not, and xfs_qm_dqalloc() then does this after calling
> xfs_bmapi_write():
>
> ASSERT((map.br_startblock != DELAYSTARTBLOCK) &&
> (map.br_startblock != HOLESTARTBLOCK));
>
> What's to prevent someone coming in between the xfs_bmapi_read()
> and *write() calls and allocating a different dquot in the same
> cluster and therefore beating the first thread to the allocation?
>
> This read/write race exists elsewhere - e.g. xfs_iomap_write_allocate
> documents it for the data path - and it has to be specifically
> handled to prevent corruption.....
Yeah, we'll need to read-read the extent map in xfs_qm_dqalloc. I I
think this is efficiently paper over by the buffer lock - we take
it right after the xfs_bmapi_write over the period of initialization
the on-disk dquots and copying the one we were called for into the
in-memory one. So while we might have been corrupting dquots all
over no one has noticed because we had a non-corrupted version
in-memory that overwrote the corrupted one again later. Uhh..
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next prev parent reply other threads:[~2013-12-05 20:53 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-12-05 15:58 [PATCH 0/5] extent list locking fixes Christoph Hellwig
2013-12-05 15:58 ` [PATCH 1/5] xfs: take the ilock around xfs_bmapi_read in xfs_zero_remaining_bytes Christoph Hellwig
2013-12-05 19:38 ` Ben Myers
2013-12-05 20:31 ` Dave Chinner
2013-12-05 20:37 ` Ben Myers
2013-12-05 20:40 ` Christoph Hellwig
2013-12-05 15:58 ` [PATCH 2/5] xfs: use xfs_ilock_map_shared in xfs_qm_dqtobp Christoph Hellwig
2013-12-05 19:46 ` Ben Myers
2013-12-05 20:41 ` Dave Chinner
2013-12-05 20:53 ` Christoph Hellwig [this message]
2013-12-05 21:03 ` Dave Chinner
2013-12-05 15:58 ` [PATCH 3/5] xfs: use xfs_ilock_map_shared in xfs_qm_dqiterate Christoph Hellwig
2013-12-05 19:48 ` Ben Myers
2013-12-05 20:45 ` Dave Chinner
2013-12-05 15:58 ` [PATCH 4/5] xfs: use xfs_ilock_map_shared in xfs_attr_get Christoph Hellwig
2013-12-05 19:57 ` Ben Myers
2013-12-05 20:59 ` Dave Chinner
2013-12-05 21:01 ` Christoph Hellwig
2013-12-05 21:05 ` Dave Chinner
2013-12-05 21:10 ` Christoph Hellwig
2013-12-05 21:17 ` Dave Chinner
2013-12-05 15:58 ` [PATCH 5/5] xfs: assert that we hold the ilock for extent map access Christoph Hellwig
2013-12-05 20:11 ` Ben Myers
2013-12-05 21:10 ` Dave Chinner
2013-12-05 21:22 ` Christoph Hellwig
2013-12-05 21:40 ` Dave Chinner
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