[PATCH] blk-cgroup: Use cond_resched() when destroy blkgs

On !PREEMPT kernel, we can get below softlockup when doing stress
testing with creating and destroying block cgroup repeatly. The
reason is it may take a long time to acquire the queue's lock in
the loop of blkcg_destroy_blkgs(), thus we can use cond_resched()
instead of cpu_relax() to avoid this issue, since the
blkcg_destroy_blkgs() is not called from atomic contexts.

[ 4757.010308] watchdog: BUG: soft lockup - CPU#11 stuck for 94s!
[ 4757.010698] Call trace:
[ 4757.010700]  blkcg_destroy_blkgs+0x68/0x150
[ 4757.010701]  cgwb_release_workfn+0x104/0x158
[ 4757.010702]  process_one_work+0x1bc/0x3f0
[ 4757.010704]  worker_thread+0x164/0x468
[ 4757.010705]  kthread+0x108/0x138

Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
---
 block/blk-cgroup.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 3465d6e..af7c0ce 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c

@@ -1028,7 +1028,7 @@ void blkcg_destroy_blkgs(struct blkcg *blkcg)
 			spin_unlock(&q->queue_lock);
 		} else {
 			spin_unlock_irq(&blkcg->lock);
-			cpu_relax();
+			cond_resched();
 			spin_lock_irq(&blkcg->lock);
 		}
 	}

-- 
1.8.3.1