Re: [PATCH] sbitmap: fix possible io hung due to lost wakeup

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



在 2022/08/03 20:15, Yu Kuai 写道:
From: Yu Kuai <yukuai3@xxxxxxxxxx>

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.

friendly ping ...

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

  previous versions:
rfc v1: https://lore.kernel.org/all/20220617141125.3024491-1-yukuai3@xxxxxxxxxx/
rfc v2: https://lore.kernel.org/all/20220619080309.1630027-1-yukuai3@xxxxxxxxxx/
rfc v3: https://lore.kernel.org/all/20220710042200.20936-1-yukuai1@xxxxxxxxxxxxxxx/
rfc v4: https://lore.kernel.org/all/20220723024122.2990436-1-yukuai1@xxxxxxxxxxxxxxx/
  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 decrementing 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;
  }





[Index of Archives]     [Linux RAID]     [Linux SCSI]     [Linux ATA RAID]     [IDE]     [Linux Wireless]     [Linux Kernel]     [ATH6KL]     [Linux Bluetooth]     [Linux Netdev]     [Kernel Newbies]     [Security]     [Git]     [Netfilter]     [Bugtraq]     [Yosemite News]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Device Mapper]

  Powered by Linux