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Re: Multi-CPU harmless lockdep on x86 while copying data

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: Multi-CPU harmless lockdep on x86 while copying data
From: Ben Myers <bpm@xxxxxxx>
Date: Mon, 10 Mar 2014 16:16:58 -0500
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>, "Michael L. Semon" <mlsemon35@xxxxxxxxx>, xfs-oss <xfs@xxxxxxxxxxx>
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20140310204647.GW6851@dastard>
References: <531BD8B9.1090400@xxxxxxxxx> <20140310025523.GV6851@dastard> <20140310103716.GA1431@xxxxxxxxxxxxx> <20140310204647.GW6851@dastard>
User-agent: Mutt/1.5.20 (2009-06-14)
Hi,

On Tue, Mar 11, 2014 at 07:46:47AM +1100, Dave Chinner wrote:
> On Mon, Mar 10, 2014 at 03:37:16AM -0700, Christoph Hellwig wrote:
> > On Mon, Mar 10, 2014 at 01:55:23PM +1100, Dave Chinner wrote:
> > > Changing the directory code to handle this sort of locking is going
> > > to require a bit of surgery. However, I can see advantages to moving
> > > directory data to the same locking strategy as regular file data -
> > > locking heirarchies are identical, directory ilock hold times are
> > > much reduced, we don't get lockdep whining about taking page faults
> > > with the ilock held, etc.
> > > 
> > > A quick hack at to demonstrate the high level, initial step of using
> > > the IOLOCK for readdir serialisation. I've done a little smoke
> > > testing on it, so it won't die immediately. It should get rid of all
> > > the nasty lockdep issues, but it doesn't start to address the deeper
> > > restructing that is needed.
> > 
> > What synchronization do we actually need from the iolock?  Pushing the
> > ilock down to where it's actually needed is a good idea either way,
> > though.
> 
> The issue is that if we push the ilock down to the just the block
> mapping routines, the directory can be modified while the readdir is
> in progress. That's the root problem that adding the ilock solved.
> Now, just pushing the ilock down to protect the bmbt lookups might
> result in a consistent lookup, but it won't serialise sanely against
> modifications.
> 
> i.e. readdir only walks one dir block at a time but
> it maps multiple blocks for readahead and keeps them in a local
> array and doesn't validate them again before issuing read o nthose
> buffers. Hence at a high level we currently have to serialise
> readdir against all directory modifications.
> 
> The only other option we might have is to completely rewrite the
> directory readahead code not to cache mappings. If we use the ilock
> purely for bmbt lookup and buffer read, then the ilock will
> serialise against modification, and the buffer lock will stabilise
> the buffer until the readdir moves to the next buffer and picks the
> ilock up again to read it.
> 
> That would avoid the need for high level serialisation, but it's a
> lot more work than using the iolock to provide the high level
> serialisation and i'm still not sure it's 100% safe. And I've got no
> idea if it would work for CXFS. Hopefully someone from SGI will
> chime in here....

Also in leaf and node formats a single modification can change multiple
buffers, so I suspect the buffer lock isn't enough serialization to maintain a
consistent directory in the face of multiple readers and writers.  The iolock
does resolve that issue.

> > > This would be a straight forward change, except for two things:
> > > filestreams and lockdep. The filestream allocator takes the
> > > directory iolock and makes assumptions about parent->child locking
> > > order of the iolock which will now be invalidated. Hence some
> > > changes to the filestreams code is needed to ensure that it never
> > > blocks on directory iolocks and deadlocks. instead it needs to fail
> > > stream associations when such problems occur.
> > 
> > I think the right fix is to stop abusing the iolock in filestreams.
> > To me it seems like a look inside fstrm_item_t should be fine
> > for what the filestreams code wants if I understand it correctly.
> > 
> > From looking over some of the filestreams code just for a few minutes
> > I get an urge to redo lots of it right now..
> 
> I get that urge from time to time, too. So far I've managed to avoid
> it.
> 
> > > @@ -1228,7 +1244,7 @@ xfs_create(
> > >    * the transaction cancel unlocking dp so don't do it explicitly in the
> > >    * error path.
> > >    */
> > > - xfs_trans_ijoin(tp, dp, XFS_ILOCK_EXCL);
> > > + xfs_trans_ijoin(tp, dp, XFS_IOLOCK_EXCL | XFS_ILOCK_EXCL);
> > 
> > What do we need the iolock on these operations for?
> 
> These are providing the high level readdir vs modification
> serialisation protection. And we have to unlock it on transaction
> commit, which is why it needs to be added to the xfs_trans_ijoin()
> calls...

Makes sense, I think.  I'm not sure what the changes to the directory code
would look like.

-Ben

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