Re: [PATCH 4/8] xfs: Move ilock before transaction start in xfs_setattr_size()
From: Jan Kara <jack@suse.cz>
Date: 2012-01-24 11:52:34
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linux-ext4, linux-fsdevel, lkml
On Tue 24-01-12 17:59:45, Dave Chinner wrote:
On Fri, Jan 20, 2012 at 09:34:42PM +0100, Jan Kara wrote:quoted
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 [off-list ref] SUSE Labs, CR _______________________________________________ xfs mailing list xfs@oss.sgi.com http://oss.sgi.com/mailman/listinfo/xfs