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

[Kemeng Shi]

on 12/2/2022 8:58 AM, Jens Axboe wrote:
> On 12/1/22 12:21?AM, Kemeng Shi wrote:
>>
>>
>> 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.
> 
> Most likely because waitqueue_active() could be racy, so a comment is
> warranted on why it's safe rather than using wq_has_sleeper().
Thanks for explanation, so the patch seems fine as comment is present
already though it doesn't mention sting "waitqueue_active" directly.
No bother anymore, this patch will be dropped as the fixed code is
stale.
Thanks again.

-- 
Best wishes
Kemeng Shi