linux-kernel - Re: [RFC, PATCH] filesystem freeze: fix sys

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Date:	Wed, 26 Aug 2009 19:38:39 +0200
From:	Christoph Hellwig <hch@....de>
To:	Fernando Luis V??zquez Cao <fernando@....ntt.co.jp>
Cc:	t-sato@...jp.nec.com, m-hamaguchi@...jp.nec.com,
	Christoph Hellwig <hch@....de>,
	Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Eric Sandeen <sandeen@...hat.com>
Subject: Re: [RFC, PATCH] filesystem freeze: fix sys_umount induced perpetual freeze

On Wed, Aug 26, 2009 at 02:00:01PM +0900, Fernando Luis V??zquez Cao wrote:
> The current locking scheme for filesystem freeze avoids races between
> freeze_bdev() and do_umount() by taking the s_umount semaphore.
> 
> If freeze_bdev() wins the race the process that invoked sys_umount
> will sleep until thaw_bdev releases the semaphore. Unfortunately, this
> will never happen because thaw_bdev bails out early the
> bd_fsfreeze_count check having failed (the count is 0).
> 
> The problem is that the block_device that ioctl_fsthaw() passes to
> thaw_bdev() is not the one that we freezed because before sleeping in
> deactivate_super() do_umount() released the dentry (dput()) and freed
> the vfs mount (free_vfsmnt()).
> 
> This patch works around this issue by checking the freeze level in
> do_umount()


This should be solved my my freeze locking rewrite:

	http://marc.info/?l=linux-fsdevel&m=124933489118480&w=2
	http://marc.info/?l=linux-fsdevel&m=124933491918517&w=2

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