linux-kernel - Re: [PATCH] vfs: don't hold s_umount over close_bdev

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Message-ID: <20100525083003.GE23411@kernel.dk>
Date:	Tue, 25 May 2010 10:30:03 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ciprian Docan <docan@...n.rutgers.edu>,
	linux-kernel@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH] vfs: don't hold s_umount over close_bdev_exclusive()
	call

On Sat, May 22 2010, Tejun Heo wrote:
> This patch fixes an obscure AB-BA deadlock in get_sb_bdev().
> 
> When a superblock is mounted more than once get_sb_bdev() calls
> close_bdev_exclusive() to drop the extra bdev reference while holding
> s_umount.  However, sb->s_umount nests inside bd_mutex during
> __invalidate_device() and close_bdev_exclusive() acquires bd_mutex
> during blkdev_put(); thus creating an AB-BA deadlock.
> 
> This condition doesn't trigger frequently.  For this condition to be
> visible to lockdep, the filesystem must occupy the whole device (as
> __invalidate_device() only grabs bd_mutex for the whole device), the
> FS must be mounted more than once and partition rescan should be
> issued while the FS is still mounted.
> 
> Fix it by dropping s_umount over close_bdev_exclusive().

Looks safe to me, since it has (as you note) an elevated ref count.

Acked-by: Jens Axboe <jens.axboe@...cle.com>


-- 
Jens Axboe

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