loopdev: fix a deadlock
authorGuo Chao <yan@linux.vnet.ibm.com>
Thu, 21 Feb 2013 23:16:45 +0000 (15:16 -0800)
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Wed, 20 Mar 2013 20:10:57 +0000 (13:10 -0700)
commit26dbc5832ef354bc3cade3c4aff16025342e6708
tree44260bce5fc4bf8c650e1069226586b70a55ae94
parentf884319c7db711bc4778a8aa4693cc444e570486
loopdev: fix a deadlock

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@linux.vnet.ibm.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Guo Chao <yan@linux.vnet.ibm.com>
Cc: M. Hindess <hindessm@uk.ibm.com>
Cc: Nikanth Karthikesan <knikanth@suse.de>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Acked-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
drivers/block/loop.c