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Re: [PATCH] xfs: use s_umount sema in xfs_sync_worker

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH] xfs: use s_umount sema in xfs_sync_worker
From: Ben Myers <bpm@xxxxxxx>
Date: Wed, 16 May 2012 12:04:03 -0500
Cc: xfs@xxxxxxxxxxx
In-reply-to: <20120516015626.GN25351@dastard>
References: <20120323174327.GU7762@xxxxxxx> <20120514203449.GE16099@xxxxxxx> <20120516015626.GN25351@dastard>
User-agent: Mutt/1.5.20 (2009-06-14)
Hey Dave,

On Wed, May 16, 2012 at 11:56:26AM +1000, Dave Chinner wrote:
> On Mon, May 14, 2012 at 03:34:49PM -0500, Ben Myers wrote:
> > I'm still hitting this on a regular basis.  Here is some analysis from a 
> > recent
> > crash dump which you may want to skip.  The fix is at the end.
> ....
> > ===================================================================
> > 
> > xfs: use s_umount sema in xfs_sync_worker
> > 
> > xfs_sync_worker checks the MS_ACTIVE flag in sb->s_flags to avoid doing work
> > during mount and unmount.  This flag can be cleared by unmount after the
> > xfs_sync_worker checks it but before the work is completed.
> 
> Then there are problems all over the place in different filesystems
> if the straight MS_ACTIVE check is not sufficient.

Eh, I won't speak to the problems in other filesystems.  ;)

MS_ACTIVE certainly isn't adequate in the case before us.

> > Protect xfs_sync_worker by using the s_umount semaphore at the read level to
> > provide exclusion with unmount while work is progressing.
> 
> I don't think that is the right fix for the given problem.
> 
> The problem is, as you've stated:
> 
> "Looks like the problem is that the sync worker is still running
> after the log has been torn down, and it calls xfs_fs_log_dummy
> which generates log traffic."

That's one problem, but we also want to protect against running this code at
mount time.  s_umount sema is the tool that can do both.  Maybe there are some
other options.

> Why did we allow a new transaction to start while/after the log was
> torn down?

> Isn't that the problem we need to fix because it leads to
> invalid entries in the physical log that might cause recovery
> failures?

> Further, any asynchronous worker thread that does
> transactions could have this same problem regardless of whether we
> are umounting or cleaning up after a failed mount, so it is not
> unique to the xfs_sync_worker....

> That is, if we've already started to tear down or torn down the log,
> we must not allow new transactions to start. Likewise, we can't
> finalise tear down the log until transactions in progress have
> completed. Using the s_umount lock here avoids then race, but it
> really is a VFS level lock not an filesystem level lock) and is,
> IMO, just papering over the real problem....

I think you have a good point here, but this isn't limited to transactions.  We
shouldn't even call xfs_log_need_covered without some protection from
xfs_fs_put_super; xfs_fs_writable doesn't cut the mustard.   I'd better have a
look at the other workqueues too.  Thanks for pointing this out.

Regards,
        Ben

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