linux-ext4 - Re: [PATCH] quota: Fix deadlock during path resolution

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Date:	Thu, 16 Sep 2010 00:52:08 +0200
From:	Christoph Hellwig <hch@....de>
To:	Jan Kara <jack@...e.cz>
Cc:	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
	reiserfs-devel@...r.kernel.org, ocfs2-devel@....oracle.com,
	Al Viro <viro@...IV.linux.org.uk>,
	Christoph Hellwig <hch@....de>, "Ted Ts'o" <tytso@....edu>,
	Joel Becker <joel.becker@...cle.com>
Subject: Re: [PATCH] quota: Fix deadlock during path resolution

On Wed, Sep 15, 2010 at 11:39:51PM +0200, Jan Kara wrote:
> As Al Viro pointed out path resolution during Q_QUOTAON calls to quotactl
> is prone to deadlocks. We hold s_umount semaphore for reading during the
> path resolution and resolution itself may need to acquire the semaphore
> for writing when e. g. autofs mountpoint is passed.
> 
> Solve the problem by performing the resolution before we get hold of the
> superblock (and thus s_umount semaphore). The whole thing is complicated
> by the fact that some filesystems (OCFS2) ignore the path argument. So to
> distinguish between filesystem which want the path and which do not we
> introduce new .quota_on_meta callback which does not get the path. OCFS2
> then uses this callback instead of old .quota_on.

FYI I have a patch that switches ocfs to share the quota_on method with
XFS which behaves the same, and consolidate the quota_off method for all
filesystems.  I think that's better than adding yet another method.

I'll rebase it and send it out soon.

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