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Subject[PATCH block/for-next] block: don't call blk_drain_queue() if elevator is not up
blk_cleanup_queue() may be called before elevator is set up on a
queue which triggers the following oops.

BUG: unable to handle kernel NULL pointer dereference at (null)
IP: [<ffffffff8125a69c>] elv_drain_elevator+0x1c/0x70
...
Pid: 830, comm: kworker/0:2 Not tainted 3.1.0-next-20111025_64+ #1590
Bochs Bochs
RIP: 0010:[<ffffffff8125a69c>] [<ffffffff8125a69c>] elv_drain_elevator+0x1c/0x70
...
Call Trace:
[<ffffffff8125da92>] blk_drain_queue+0x42/0x70
[<ffffffff8125db90>] blk_cleanup_queue+0xd0/0x1c0
[<ffffffff81469640>] md_free+0x50/0x70
[<ffffffff8126f43b>] kobject_release+0x8b/0x1d0
[<ffffffff81270d56>] kref_put+0x36/0xa0
[<ffffffff8126f2b7>] kobject_put+0x27/0x60
[<ffffffff814693af>] mddev_delayed_delete+0x2f/0x40
[<ffffffff81083450>] process_one_work+0x100/0x3b0
[<ffffffff8108527f>] worker_thread+0x15f/0x3a0
[<ffffffff81089937>] kthread+0x87/0x90
[<ffffffff81621834>] kernel_thread_helper+0x4/0x10

Fix it by making blk_cleanup_queue() check whether q->elevator is set
up before invoking blk_drain_queue.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-and-tested-by: Jiri Slaby <jslaby@suse.cz>
---
block/blk-core.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index f658711..f43c8a5 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -407,8 +407,13 @@ void blk_cleanup_queue(struct request_queue *q)
spin_unlock_irq(lock);
mutex_unlock(&q->sysfs_lock);

- /* drain all requests queued before DEAD marking */
- blk_drain_queue(q, true);
+ /*
+ * Drain all requests queued before DEAD marking. The caller might
+ * be trying to tear down @q before its elevator is initialized, in
+ * which case we don't want to call into draining.
+ */
+ if (q->elevator)
+ blk_drain_queue(q, true);

/* @q won't process any more request, flush async actions */
del_timer_sync(&q->backing_dev_info.laptop_mode_wb_timer);

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