On Wed, 19 Oct 2022, Greg Kroah-Hartman wrote:
> From: Yu Kuai <yukuai3@xxxxxxxxxx>
>
> [ Upstream commit 040b83fcecfb86f3225d3a5de7fd9b3fbccf83b4 ]
>
> 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>
> Reviewed-by: Jan Kara <jack@xxxxxxx>
> Link: https://lore.kernel.org/r/20220803121504.212071-1-yukuai1@xxxxxxxxxxxxxxx
> Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
I have no authority on linux-block, but I'll say NAK to this one
(and 517/862), and let Jens and Jan overrule me if they disagree.
This was the first of several 6.1-rc1 commits which had given me lost
wakeups never suffered before; was not tagged Cc stable; and (unless I've
missed it on lore) never had AUTOSEL posted to linux-block or linux-kernel.
Hugh
> ---
> lib/sbitmap.c | 55 ++++++++++++++++++++++++++++++---------------------
> 1 file changed, 33 insertions(+), 22 deletions(-)
>
> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> index 29eb0484215a..1f31147872e6 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;
> }
> --
> 2.35.1
>
>
>
>
