linux-ext4 - Re: [PATCH] fs: fix filesystem_sync vs write race on rw=>ro remount

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Message-ID: <20100209152856.GB15318@atrey.karlin.mff.cuni.cz>
Date:	Tue, 9 Feb 2010 16:28:57 +0100
From:	Jan Kara <jack@...e.cz>
To:	Dave Chinner <david@...morbit.com>
Cc:	Al Viro <viro@...IV.linux.org.uk>,
	Dmitry Monakhov <dmonakhov@...nvz.org>,
	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: fix filesystem_sync vs write race on rw=>ro remount

> On Sun, Jan 24, 2010 at 09:37:07PM +0000, Al Viro wrote:
> > On Mon, Jan 25, 2010 at 12:15:51AM +0300, Dmitry Monakhov wrote:
> > 
> > > > It's not a solution.  You get an _attempted_ remount ro making writes
> > > > fail, even if it's going to be unsuccessful.  No go...
> > > We have two options for new writers:
> > > 1) Fail it via -EROFS
> > >    Yes, remount may fail, but it is really unlikely.
> > > 2) Defer(block) new writers on until we complete or fail remount
> > >    for example like follows. Do you like second solution ?
> > 
> > Umm...  I wonder what the locking implications would be...  Frankly,
> > I suspect that what we really want is this:
> > 	* per-superblock write count of some kind, bumped when we decide
> > that writeback is inevitable and dropped when we are done with it (the
> > same thing goes for async part of unlink(), etc.)
> > 	* fs_may_remount_ro() checking that write count
> > So basically we try to push those short-term writers to completion and
> > if new ones had come while we'd been doing that (or some are really
> > stuck) we fail remount with -EBUSY.
> 
> Perhaps we could utilise the filesystem freeze infrastructure - it
> already has hooks for intercepting new writers and modifcations,
> and filesystems have to flush any current modifications before the freeze
> completes. It sounds very similar to the requirements needed here...
  There are filesystems (e.g. ext2 or UDF) which don't support freezing so it's not
an option at least short term...

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