Re: [PATCH 4/8] xfs: Move ilock before transaction start in xfs_setattr_size()

Jan Kara
2012-01-24T11:52:38Z

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@suse.cz>
SUSE Labs, CR