Date:   Sat, 23 Jul 2022 10:41:22 +0800
From:   Yu Kuai <yukuai1@...weicloud.com>
To:     jack@...e.cz, axboe@...nel.dk, osandov@...com
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        yukuai3@...wei.com, yukuai1@...weicloud.com, yi.zhang@...wei.com
Subject: [PATCH RFC v4] sbitmap: fix possible io hung due to lost wakeup

From: Yu Kuai <yukuai3@...wei.com>

There are two problems can lead to lost wakeup:

1) invalid wakeup on the wrong waitqueue:

For example, 2 * wake_batch tags are put, while only wake_batch threads
are woken:

__sbq_wake_up
 atomic_cmpxchg -> reset wait_cnt
			__sbq_wake_up -> decrease wait_cnt
			...
			__sbq_wake_up -> wait_cnt is decreased to 0 again
			 atomic_cmpxchg
			 sbq_index_atomic_inc -> increase wake_index
			 wake_up_nr -> wake up and waitqueue might be empty
 sbq_index_atomic_inc -> increase again, one waitqueue is skipped
 wake_up_nr -> invalid wake up because old wakequeue might be empty

To fix the problem, increasing 'wake_index' before resetting 'wait_cnt'.

2) 'wait_cnt' can be decreased while waitqueue is empty

As pointed out by Jan Kara, following race is possible:

CPU1				CPU2
__sbq_wake_up			 __sbq_wake_up
 sbq_wake_ptr()			 sbq_wake_ptr() -> the same
 wait_cnt = atomic_dec_return()
 /* decreased to 0 */
 sbq_index_atomic_inc()
 /* move to next waitqueue */
 atomic_set()
 /* reset wait_cnt */
 wake_up_nr()
 /* wake up on the old waitqueue */
				 wait_cnt = atomic_dec_return()
				 /*
				  * decrease wait_cnt in the old
				  * waitqueue, while it can be
				  * empty.
				  */

Fix the problem by waking up before updating 'wake_index' and
'wait_cnt'.

With this patch, noted that 'wait_cnt' is still decreased in the old
empty waitqueue, however, the wakeup is redirected to a active waitqueue,
and the extra decrement on the old empty waitqueue is not handled.

Fixes: 88459642cba4 ("blk-mq: abstract tag allocation out into sbitmap library")
Signed-off-by: Yu Kuai <yukuai3@...wei.com>
---
Changes in v4:
 - remove patch 1, which improve fairness with overhead
 - merge patch2 and patch 3
Changes in v3:
 - rename patch 2, and add some comments.
 - add patch 3, which fixes a new issue pointed out by Jan Kara.
Changes in v2:
 - split to spearate patches for different problem.
 - add fix tag

 previous versions:
v1: https://lore.kernel.org/all/20220617141125.3024491-1-yukuai3@huawei.com/
v2: https://lore.kernel.org/all/20220619080309.1630027-1-yukuai3@huawei.com/
v3: https://lore.kernel.org/all/20220710042200.20936-1-yukuai1@huaweicloud.com/
 lib/sbitmap.c | 55 ++++++++++++++++++++++++++++++---------------------
 1 file changed, 33 insertions(+), 22 deletions(-)

diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index 29eb0484215a..1aa55806f6a5 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -611,32 +611,43 @@ static bool __sbq_wake_up(struct sbitmap_queue *sbq)
 		return false;
 
 	wait_cnt = atomic_dec_return(&ws->wait_cnt);
-	if (wait_cnt <= 0) {
-		int ret;
+	/*
+	 * For concurrent callers of this, callers should call this function
+	 * again to wakeup a new batch on a different 'ws'.
+	 */
+	if (wait_cnt < 0 || !waitqueue_active(&ws->wait))
+		return true;
 
-		wake_batch = READ_ONCE(sbq->wake_batch);
+	if (wait_cnt > 0)
+		return false;
 
-		/*
-		 * Pairs with the memory barrier in sbitmap_queue_resize() to
-		 * ensure that we see the batch size update before the wait
-		 * count is reset.
-		 */
-		smp_mb__before_atomic();
+	wake_batch = READ_ONCE(sbq->wake_batch);
 
-		/*
-		 * For concurrent callers of this, the one that failed the
-		 * atomic_cmpxhcg() race should call this function again
-		 * to wakeup a new batch on a different 'ws'.
-		 */
-		ret = atomic_cmpxchg(&ws->wait_cnt, wait_cnt, wake_batch);
-		if (ret == wait_cnt) {
-			sbq_index_atomic_inc(&sbq->wake_index);
-			wake_up_nr(&ws->wait, wake_batch);
-			return false;
-		}
+	/*
+	 * Wake up first in case that concurrent callers decrease wait_cnt
+	 * while waitqueue is empty.
+	 */
+	wake_up_nr(&ws->wait, wake_batch);
 
-		return true;
-	}
+	/*
+	 * Pairs with the memory barrier in sbitmap_queue_resize() to
+	 * ensure that we see the batch size update before the wait
+	 * count is reset.
+	 *
+	 * Also pairs with the implicit barrier between becrementing wait_cnt
+	 * and checking for waitqueue_active() to make sure waitqueue_active()
+	 * sees result of the wakeup if atomic_dec_return() has seen the result
+	 * of atomic_set().
+	 */
+	smp_mb__before_atomic();
+
+	/*
+	 * Increase wake_index before updating wait_cnt, otherwise concurrent
+	 * callers can see valid wait_cnt in old waitqueue, which can cause
+	 * invalid wakeup on the old waitqueue.
+	 */
+	sbq_index_atomic_inc(&sbq->wake_index);
+	atomic_set(&ws->wait_cnt, wake_batch);
 
 	return false;
 }
-- 
2.31.1

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