linux-kernel - [ 79/82] loopdev: fix a deadlock

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Date:	Mon, 18 Mar 2013 04:23:03 +0000
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	akpm@...ux-foundation.org, Guo Chao <yan@...ux.vnet.ibm.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	"M. Hindess" <hindessm@...ibm.com>,
	Nikanth Karthikesan <knikanth@...e.de>,
	Jens Axboe <axboe@...nel.dk>
Subject: [ 79/82] loopdev: fix a deadlock

3.2-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Guo Chao <yan@...ux.vnet.ibm.com>

commit 5370019dc2d2c2ff90e95d181468071362934f3a upstream.

bd_mutex and lo_ctl_mutex can be held in different order.

Path #1:

blkdev_open
 blkdev_get
  __blkdev_get (hold bd_mutex)
   lo_open (hold lo_ctl_mutex)

Path #2:

blkdev_ioctl
 lo_ioctl (hold lo_ctl_mutex)
  lo_set_capacity (hold bd_mutex)

Lockdep does not report it, because path #2 actually holds a subclass of
lo_ctl_mutex.  This subclass seems creep into the code by mistake.  The
patch author actually just mentioned it in the changelog, see commit
f028f3b2 ("loop: fix circular locking in loop_clr_fd()"), also see:

	http://marc.info/?l=linux-kernel&m=123806169129727&w=2

Path #2 hold bd_mutex to call bd_set_size(), I've protected it
with i_mutex in a previous patch, so drop bd_mutex at this site.

Signed-off-by: Guo Chao <yan@...ux.vnet.ibm.com>
Cc: Alexander Viro <viro@...iv.linux.org.uk>
Cc: Guo Chao <yan@...ux.vnet.ibm.com>
Cc: M. Hindess <hindessm@...ibm.com>
Cc: Nikanth Karthikesan <knikanth@...e.de>
Cc: Jens Axboe <axboe@...nel.dk>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Jens Axboe <axboe@...nel.dk>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/block/loop.c |    2 --
 1 file changed, 2 deletions(-)

--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1262,11 +1262,9 @@ static int loop_set_capacity(struct loop
 	/* the width of sector_t may be narrow for bit-shift */
 	sz = sec;
 	sz <<= 9;
-	mutex_lock(&bdev->bd_mutex);
 	bd_set_size(bdev, sz);
 	/* let user-space know about the new size */
 	kobject_uevent(&disk_to_dev(bdev->bd_disk)->kobj, KOBJ_CHANGE);
-	mutex_unlock(&bdev->bd_mutex);
 
  out:
 	return err;


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