Re: [PATCH 1/5] sbitmap: don't consume nr for inactive waitqueue to avoid lost wakeups

From: Kemeng Shi
Date: Thu Dec 01 2022 - 02:21:17 EST




on 12/1/2022 12:54 PM, Kemeng Shi wrote:
> If we decremented queue without waiters, we should not decremente freed
> bits number "nr", or all "nr" could be consumed in a empty queue and no
> wakeup will be called.
> Currently, for case "wait_cnt > 0", "nr" will not be decremented if we
> decremented queue without watiers and retry is returned to avoid lost
> wakeups. However for case "wait_cnt == 0", "nr" will be decremented
> unconditionally and maybe decremented to zero. Although retry is
> returned by active state of queue, it's not actually executed for "nr"
> is zero.
>
> Fix this by only decrementing "nr" for active queue when "wait_cnt ==
> 0". After this fix, "nr" will always be non-zero when we decremented
> inactive queue for case "wait_cnt == 0", so the need to retry could
> be returned by "nr" and active state of waitqueue returned for the same
> purpose is not needed.
>
> Signed-off-by: Kemeng Shi <shikemeng@xxxxxxxxxx>
> ---
> lib/sbitmap.c | 13 ++++++-------
> 1 file changed, 6 insertions(+), 7 deletions(-)
>
> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> index 7280ae8ca88c..e40759bcf821 100644
> --- a/lib/sbitmap.c
> +++ b/lib/sbitmap.c
> @@ -604,7 +604,6 @@ static bool __sbq_wake_up(struct sbitmap_queue *sbq, int *nr)
> struct sbq_wait_state *ws;
> unsigned int wake_batch;
> int wait_cnt, cur, sub;
> - bool ret;
>
> if (*nr <= 0)
> return false;
> @@ -632,15 +631,15 @@ static bool __sbq_wake_up(struct sbitmap_queue *sbq, int *nr)
> if (wait_cnt > 0)
> return !waitqueue_active(&ws->wait);
>
> - *nr -= sub;
> -
> /*
> * When wait_cnt == 0, we have to be particularly careful as we are
> * responsible to reset wait_cnt regardless whether we've actually
> - * woken up anybody. But in case we didn't wakeup anybody, we still
> - * need to retry.
> + * woken up anybody. But in case we didn't wakeup anybody, we should
> + * not consume nr and need to retry to avoid lost wakeups.
> */
> - ret = !waitqueue_active(&ws->wait);
There is a warnning reported by checkpatch.pl which is "WARNING:waitqueue_active
without comment" but I don't know why.
> + if (waitqueue_active(&ws->wait))
> + *nr -= sub;
> +
> wake_batch = READ_ONCE(sbq->wake_batch);
>
> /*
> @@ -669,7 +668,7 @@ static bool __sbq_wake_up(struct sbitmap_queue *sbq, int *nr)
> sbq_index_atomic_inc(&sbq->wake_index);
> atomic_set(&ws->wait_cnt, wake_batch);
>
> - return ret || *nr;
> + return *nr;
> }
>
> void sbitmap_queue_wake_up(struct sbitmap_queue *sbq, int nr)
>
Besides, there are some git config problems for my huaweicloud email, I will send
patchset with huaweicloud email when I fix them.

Thanks.
--
Best wishes
Kemeng Shi