commit 4f331f01b9c43bf001d3ffee578a97a1e0633eac
author Tejun Heo <tj@kernel.org> Tue Jul 20 15:18:07 2010 -0700
committer Al Viro <viro@zeniv.linux.org.uk> Mon Aug 09 16:48:59 2010 -0400
tree 77cd690ab7af2624e3fd7932563f6dc0f5d6441a
parent 719f2c879f4dda7d7f303bd387d37cd96db29d31

vfs: don't hold s_umount over close_bdev_exclusive() call

Fix 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().

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Ciprian Docan <docan@eden.rutgers.edu>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>