linux-ext4 - Re: [BUG] ext4: cannot unfreeze a filesystem due to a deadlock

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Message-ID: <20110215170352.GE4255@thunk.org>
Date:	Tue, 15 Feb 2011 12:03:52 -0500
From:	Ted Ts'o <tytso@....edu>
To:	Jan Kara <jack@...e.cz>
Cc:	Masayoshi MIZUMA <m.mizuma@...fujitsu.com>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [BUG] ext4: cannot unfreeze a filesystem due to a deadlock

On Tue, Feb 15, 2011 at 05:06:30PM +0100, Jan Kara wrote:
> Thanks for detailed analysis. Indeed this is a bug. Whenever we do IO
> under s_umount semaphore, we are prone to deadlock like the one you
> describe above.

One of the fundamental problems here is that the freeze and thaw
routines are using down_write(&sb->s_umount) for two purposes.  The
first is to prevent the resume/thaw from racing with a umount (which
it could do just as well by taking a read lock), but the second is to
prevent the resume/thaw code from racing with itself.  That's the core
fundamental problem here.

So I think we can solve this by introduce a new mutex, s_freeze, and
having the the resume/thaw first take the s_freeze mutex and then
second take a read lock on the s_umount.

						- Ted
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