linux-kernel - Re: [PATCH] deadlock with suspend and quotas

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Date:	Tue, 29 Nov 2011 12:11:36 +0100
From:	Jan Kara <jack@...e.cz>
To:	Mikulas Patocka <mpatocka@...hat.com>
Cc:	Jan Kara <jack@...e.cz>, Valerie Aurora <val@...consulting.com>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	dm-devel@...hat.com,
	Christopher Chaltain <christopher.chaltain@...onical.com>,
	esandeen@...hat.com, Surbhi Palande <csurbhi@...il.com>
Subject: Re: [PATCH] deadlock with suspend and quotas

On Tue 29-11-11 06:06:21, Mikulas Patocka wrote:
> On Tue 29-11-11 11:19:01, Jan Kara wrote:
> >   Hi,
> > 
> > On Mon 28-11-11 18:32:18, Mikulas Patocka wrote:
> > > > Hi
> > > > 
> > > > Where can I get that patch set?
> > > > 
> > > > We are experiencing other similar deadlocks on RHEL-6, caused by sync or 
> > > > background writeback (these code paths take s_umount and wait trying to do 
> > > > I/O), but I wasn't able to reproduce these deadlocks on upstream kernel? 
> > > > Are there other known deadlock possibilities?
> > > 
> > > I found some patch named "[RFC PATCH 1/3] VFS: Fix s_umount thaw/write 
> > > deadlock" (I couldn't find the next two parts of the patch in the 
> > > archives). And the patch looks wrong:
> >   Yes, that seems to be the series. I generally agree with you that the
> > last iteration still had some problems and some changes were requested.
> > That's why it's not merged yet after all...
> > 
> > > - down_read_trylock(&sb->s_umount) doesn't fix anything. The lock is not 
> > > held when the filesystem is frozen and it is taken for write when thawing. 
> > > Consequently, any task can succeed with down_read_trylock(&sb->s_umount) 
> > > on a frozen filesystem and if this tasks attempts to do an I/O that is 
> > > waiting for thaw, it may still deadlock.
> >   Agreed.
> > 
> > > - skipping sync on frozen filesystem violates sync semantics. 
> > > Applications, such as databases, assume that when sync finishes, data were 
> > > written to stable storage. If we skip sync when the filesystem is frozen, 
> > > we can cause data corruption in these applications (if the system crashes 
> > > after we skipped a sync).
> >   Here I don't agree. Filesystem must guarantee there are no dirty data on
> > a frozen filesystem.
> 
> This is technically impossible to achieve on ext2, fat or other 
> non-transactional filesystems. These filesystems have no locks around code 
> paths that set data or inodes dirty. And you still need working sync for 
> ext2. So the best thing to do in sync is to wait until the filesystem is 
> unfrozen.
  Then suspend is effectively unsupported on the filesystem and should
return EOPNOTSUPP? At least that's what I'd expect...

									Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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