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Commit 4f331f01 authored by Tejun Heo's avatar Tejun Heo Committed by Al Viro
Browse files

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: default avatarTejun Heo <tj@kernel.org>
Reported-by: default avatarCiprian Docan <docan@eden.rutgers.edu>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: default avatarJens Axboe <axboe@kernel.dk>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
parent 719f2c87

fs/super.c

+9 −0
Original line number Diff line number Diff line
@@ -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];