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

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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>
This commit is contained in:
Tejun Heo 2010-07-20 15:18:07 -07:00 committed by Al Viro
parent 719f2c879f
commit 4f331f01b9
1 changed files with 9 additions and 0 deletions
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9
fs/super.c
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@ -773,7 +773,16 @@ int get_sb_bdev(struct file_system_type *fs_type,
goto error_bdev;
}
/*
* s_umount nests inside bd_mutex during
* __invalidate_device(). close_bdev_exclusive()
* acquires bd_mutex and can't be called under
* s_umount. Drop s_umount temporarily. This is safe
* as we're holding an active reference.
*/
up_write(&s->s_umount);
close_bdev_exclusive(bdev, mode);
down_write(&s->s_umount);
} else {
char b[BDEVNAME_SIZE];