[PATCH] loop: queue_lock NULL pointer derefence in blk_throtl_exit (v3)

[Sergey Senozhatsky <sergey.senozhatsky@gmail.com>]

Performing
$ sudo mount -o loop -o umask=0 /dev/sdb1 /mnt/
mount: wrong fs type, bad option, bad superblock on /dev/loop0,
       missing codepage or helper program, or other error
       In some cases useful info is found in syslog - try
       dmesg | tail  or so

$ sudo modprobe -r loop

results in oops:       

 BUG: unable to handle kernel NULL pointer dereference at 0000000000000004
 IP: [<ffffffff812479d4>] do_raw_spin_lock+0x14/0x122
 Process modprobe (pid: 6189, threadinfo ffff88009a898000, task
 ffff880154a88000)                                                        
 Call Trace:                                         
  [<ffffffff81486788>] _raw_spin_lock_irq+0x4a/0x51                         
  [<ffffffff8123404b>] ? blk_throtl_exit+0x3b/0xa0
  [<ffffffff8105b120>] ? cancel_delayed_work_sync+0xd/0xf
  [<ffffffff8123404b>] blk_throtl_exit+0x3b/0xa0   
  [<ffffffff81229bc8>] blk_release_queue+0x21/0x65
  [<ffffffff8123bb06>] kobject_release+0x51/0x66         
  [<ffffffff8123bab5>] ? kobject_release+0x0/0x66
  [<ffffffff8123ce1e>] kref_put+0x43/0x4d         
  [<ffffffff8123ba27>] kobject_put+0x47/0x4b    
  [<ffffffff8122717c>] blk_cleanup_queue+0x56/0x5b
  [<ffffffffa01c3824>] loop_exit+0x68/0x844 [loop]
  [<ffffffff8107cccc>] sys_delete_module+0x1e8/0x25b
  [<ffffffff814864c9>] ? trace_hardirqs_on_thunk+0x3a/0x3f
  [<ffffffff81002112>] system_call_fastpath+0x16/0x1b


because of an attempt to acquire NULL queue_lock.   
Added the same lines as in blk_queue_make_request -       
`fall back to embedded per-queue lock' - when call blk_release_queue on
allocated but not initialized queue.

v2: According to comment by Vivek Goyal, queue_lock NULL check and fix moved
out from loop driver code to blk_release_queue, which is more general
approach.                                        
         
         
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>

---

 block/blk-sysfs.c |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)

diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 41fb691..b8ca1f7 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -471,6 +471,12 @@ static void blk_release_queue(struct kobject *kobj)
 
 	blk_sync_queue(q);
 
+	/* It's possible that blk_release_queue will be called on allocated
+	 * but never initilalized queue. Fall back to our embedded per-queue 
+	 * locks in this case. */
+	if (!q->queue_lock)
+		q->queue_lock = &q->__queue_lock;
+
 	blk_throtl_exit(q);
 
 	if (rl->rq_pool)