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    SubjectRe: [PATCH 6.0 479/862] sbitmap: fix possible io hung due to lost wakeup
    On Wed, 19 Oct 2022, Greg Kroah-Hartman wrote:

    > From: Yu Kuai <yukuai3@huawei.com>
    >
    > [ 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@huawei.com>
    > Reviewed-by: Jan Kara <jack@suse.cz>
    > Link: https://lore.kernel.org/r/20220803121504.212071-1-yukuai1@huaweicloud.com
    > Signed-off-by: Jens Axboe <axboe@kernel.dk>
    > Signed-off-by: Sasha Levin <sashal@kernel.org>

    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
    >
    >
    >
    >

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