linux-kernel - Re: [PATCH vfs-2.6:for-next] vfs: remount

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Message-ID: <20090506064827.GR8633@ZenIV.linux.org.uk>
Date:	Wed, 6 May 2009 07:48:27 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Alessio Igor Bogani <abogani@...ware.it>,
	Jonathan Corbet <corbet@....net>,
	Fr??d??ric Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	LKML <linux-kernel@...r.kernel.org>,
	LFSDEV <linux-fsdevel@...r.kernel.org>,
	Matthew Wilcox <matthew@....cx>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH vfs-2.6:for-next] vfs: remount_fs BKL pushdown

On Wed, May 06, 2009 at 02:26:30AM -0400, Christoph Hellwig wrote:
> On Wed, May 06, 2009 at 04:51:38AM +0100, Al Viro wrote:
> > Actually, I'm not sure that you are right.  Especially if we go for your
> > "always hold s_umount for sync_filesystem()"; in that case we are guaranteed
> > that we'll have an exclusion between ->write_super() and that sucker, so
> > there's no reason to push it down into filesystems that do not use lock_super()
> 
> The interesting cases are locking against internal s_lock which at least
> extN needs or ->write_super.  And I'd really be rather safe than sorry
> and audit individual filesystems than introducing bug in an obscure one.

write_super() can *not* get contention against remount.  That's the point.
And other that write_super, we have very few filesystems that even mention
lock_super() anywhere.  Yes, ext3 and ext4.  Also fat, sysv, ufs and hpfs.
That's it.  Compare with the number of suckers that have write_super()
and especially remount_fs()...
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