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Re: [PATCH 4/8] xfs: Move ilock before transaction start in xfs_setattr_

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH 4/8] xfs: Move ilock before transaction start in xfs_setattr_size()
From: Jan Kara <jack@xxxxxxx>
Date: Tue, 24 Jan 2012 12:52:34 +0100
Cc: Jan Kara <jack@xxxxxxx>, linux-fsdevel@xxxxxxxxxxxxxxx, Eric Sandeen <sandeen@xxxxxxxxxxx>, Dave Chinner <dchinner@xxxxxxxxxx>, Surbhi Palande <csurbhi@xxxxxxxxx>, Kamal Mostafa <kamal@xxxxxxxxxxxxx>, Christoph Hellwig <hch@xxxxxxxxxxxxx>, LKML <linux-kernel@xxxxxxxxxxxxxxx>, xfs@xxxxxxxxxxx, linux-ext4@xxxxxxxxxxxxxxx, Ben Myers <bpm@xxxxxxx>, Alex Elder <elder@xxxxxxxxxx>
In-reply-to: <20120124065945.GL15102@dastard>
References: <1327091686-23177-1-git-send-email-jack@xxxxxxx> <1327091686-23177-5-git-send-email-jack@xxxxxxx> <20120124065945.GL15102@dastard>
User-agent: Mutt/1.5.20 (2009-06-14)
On Tue 24-01-12 17:59:45, Dave Chinner wrote:
> On Fri, Jan 20, 2012 at 09:34:42PM +0100, Jan Kara wrote:
> > In xfs we first take ilock and start transaction afterwards.
> 
> The correct order is to allocate the transaction, reserve the space
> for it and then take the ilock.  We cannot hold the ilock over the
> transaction reservation because that can deadlock the journal.
>
> That is, to make space for the new transaction reservation, we may
> need to take the ilock to flush the inode and allow the journal tail
> to move forwards to make space for the new transaction.  If we
> already hold the ilock, then it can't be flushed, we can't make
> space available in the journal and hence deadlock.
  Thanks for clarification!

> Maybe you confused the ilock vs the iolock.  We can hold the iolock
> over the trans alloc/reserve because that lock is not required to
> move the tail of the journal, so the deadlock doesn't exist.
  Ups! I now had a look at what xfs_rw_ilock() does. I always thought it's
just a plain rw semaphore and now I see it takes several locks depending on
the argument. Ugh, a bit surprising for XFS newcomer as me ;) But now
things become clearer so I fix my patches with this new knowledge in mind.
So just disregard my locking comments. They were likely bogus.

                                                                Honza
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR

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