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.
Fixes: 88459642cba4 ("blk-mq: abstract tag allocation out into sbitmap library")
Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
---
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@xxxxxxxxxx/
v2: https://lore.kernel.org/all/20220619080309.1630027-1-yukuai3@xxxxxxxxxx/
v3: https://lore.kernel.org/all/20220710042200.20936-1-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 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
