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Re: [PATCH 07/16] xfs: convert inode cache lookups to use RCU locking

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH 07/16] xfs: convert inode cache lookups to use RCU locking
From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
Date: Tue, 9 Nov 2010 21:12:42 -0800
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>, xfs@xxxxxxxxxxx, eric.dumazet@xxxxxxxxx
In-reply-to: <20101109050417.GI2715@dastard>
References: <1289206519-18377-1-git-send-email-david@xxxxxxxxxxxxx> <1289206519-18377-8-git-send-email-david@xxxxxxxxxxxxx> <20101108230929.GA13299@xxxxxxxxxxxxx> <20101109033628.GN4032@xxxxxxxxxxxxxxxxxx> <20101109050417.GI2715@dastard>
Reply-to: paulmck@xxxxxxxxxxxxxxxxxx
User-agent: Mutt/1.5.20 (2009-06-14)
On Tue, Nov 09, 2010 at 04:04:17PM +1100, Dave Chinner wrote:
> On Mon, Nov 08, 2010 at 07:36:28PM -0800, Paul E. McKenney wrote:
> > On Mon, Nov 08, 2010 at 06:09:29PM -0500, Christoph Hellwig wrote:
> > > This patch generally looks good to me, but with so much RCU magic I'd 
> > > prefer
> > > if Paul & Eric could look over it.
> > 
> > Is there a git tree, tarball, or whatever? 
> 
> git://git.kernel.org/pub/scm/linux/kernel/git/dgc/xfsdev.git working

Thank you -- I have downloaded this and will look it over.

Once the C++ guys get done grilling me on memory-model issues...

> contains the series that this patch is in.
> 
> > For example, I don't see
> > how this patch handles the case of an inode being freed just as an RCU
> > reader gains a reference to it,
> 
> XFS_IRECLAIM flag is set on inodes as they transition into the
> reclaim state long before they are freed. The XFS_IRECLAIM flag is left there 
> once
> freed. Hence lookups in xfs_iget_cache_hit() will see this.
> 
> If the inode has been reallocated, the inode number will not yet be
> set, or the inode state will have changed to XFS_INEW, both of which
> xfs_iget_cache_hit() will also reject.
> 
> > but then reallocated as some other inode
> > (so that ->ino is nonzero) before the RCU reader gets a chance to actually
> > look at the inode.
> 
> XFS_INEW is not cleared until well after a new ->i_ino is set, so
> the lookup should find trip over XFS_INEW in that case. I think that
> I may need to move the inode number check under the i_flags_lock
> after validating the flags - more to check that we've got the
> correct inode than to validate we have a freed inode.

OK, this sounds promising.  Of course, the next question is "how quickly
can the inode number be available for reuse?"

> > But such a check might well be in the code that this
> > patch didn't change...
> 
> Yeah, most of the XFS code is already in a form compatible with such
> RCU use because inodes have always had a quiescent "reclaimable"
> state between active and reclaim (XFS_INEW -> active ->
> XFS_IRECLAIMABLE -> XFS_IRECLAIM) where the inode can be reused
> before being freed. The result is that lookups have always had to
> handle races with inodes that have just transitioned into the
> XFS_IRECLAIM state and hence cannot be immediately reused...

Cool!!!

                                                        Thanx, Paul

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