[PATCH 3.18 017/105] dm kcopyd: avoid softlockup in run_complete_job

[Greg Kroah-Hartman]

3.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: John Pittman <jpittman@xxxxxxxxxx>

[ Upstream commit 784c9a29e99eb40b842c29ecf1cc3a79e00fb629 ]

It was reported that softlockups occur when using dm-snapshot ontop of
slow (rbd) storage.  E.g.:

[ 4047.990647] watchdog: BUG: soft lockup - CPU#10 stuck for 22s! [kworker/10:23:26177]
...
[ 4048.034151] Workqueue: kcopyd do_work [dm_mod]
[ 4048.034156] RIP: 0010:copy_callback+0x41/0x160 [dm_snapshot]
...
[ 4048.034190] Call Trace:
[ 4048.034196]  ? __chunk_is_tracked+0x70/0x70 [dm_snapshot]
[ 4048.034200]  run_complete_job+0x5f/0xb0 [dm_mod]
[ 4048.034205]  process_jobs+0x91/0x220 [dm_mod]
[ 4048.034210]  ? kcopyd_put_pages+0x40/0x40 [dm_mod]
[ 4048.034214]  do_work+0x46/0xa0 [dm_mod]
[ 4048.034219]  process_one_work+0x171/0x370
[ 4048.034221]  worker_thread+0x1fc/0x3f0
[ 4048.034224]  kthread+0xf8/0x130
[ 4048.034226]  ? max_active_store+0x80/0x80
[ 4048.034227]  ? kthread_bind+0x10/0x10
[ 4048.034231]  ret_from_fork+0x35/0x40
[ 4048.034233] Kernel panic - not syncing: softlockup: hung tasks

Fix this by calling cond_resched() after run_complete_job()'s callout to
the dm_kcopyd_notify_fn (which is dm-snap.c:copy_callback in the above
trace).

Signed-off-by: John Pittman <jpittman@xxxxxxxxxx>
Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx>
Signed-off-by: Sasha Levin <alexander.levin@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 drivers/md/dm-kcopyd.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/drivers/md/dm-kcopyd.c
+++ b/drivers/md/dm-kcopyd.c
@@ -454,6 +454,8 @@ static int run_complete_job(struct kcopy
 	if (atomic_dec_and_test(&kc->nr_jobs))
 		wake_up(&kc->destroyq);
 
+	cond_resched();
+
 	return 0;
 }