linux-kernel - [PATCH 3/7] loopdev: fix a dead lock

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Date:	Fri, 25 Jan 2013 18:56:18 +0800
From:	Guo Chao <yan@...ux.vnet.ibm.com>
To:	axboe@...nel.dk
Cc:	linux-kernel@...r.kernel.org
Subject: [PATCH 3/7] loopdev: fix a dead lock

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, 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>
---
 drivers/block/loop.c |    2 --
 1 file changed, 2 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index ae12512..7f80653 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1285,11 +1285,9 @@ 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;
-- 
1.7.9.5

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