linux-kernel - [PATCH 025/150] loopdev: fix a deadlock

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Message-Id: <1364311249-14454-26-git-send-email-luis.henriques@canonical.com>
Date:	Tue, 26 Mar 2013 15:18:44 +0000
From:	Luis Henriques <luis.henriques@...onical.com>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Cc:	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>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Luis Henriques <luis.henriques@...onical.com>
Subject: [PATCH 025/150] loopdev: fix a deadlock

3.5.7.9 -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: Luis Henriques <luis.henriques@...onical.com>
---
 drivers/block/loop.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 3bba655..11e702c 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1263,11 +1263,9 @@ static int loop_set_capacity(struct loop_device *lo, struct block_device *bdev)
 	/* 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;
-- 
1.8.1.2

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