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Message-ID: <4fbc3052-b9b7-607d-1b8c-6ad8b21dfc3c@huawei.com>
Date: Wed, 22 Jun 2022 11:58:00 +0800
From: Yu Kuai <yukuai3@...wei.com>
To: Jan Kara <jack@...e.cz>
CC: <axboe@...nel.dk>, <ming.lei@...hat.com>,
<linux-block@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
<yi.zhang@...wei.com>
Subject: Re: [PATCH RFC -next] sbitmap: fix possible io hung due to lost
wakeups
在 2022/06/21 1:02, Jan Kara 写道:
> On Mon 20-06-22 21:44:16, Yu Kuai wrote:
>> 在 2022/06/20 20:48, Jan Kara 写道:
>>> On Mon 20-06-22 14:24:13, Jan Kara wrote:
>>>> On Fri 17-06-22 22:11:25, Yu Kuai wrote:
>>>>> Currently, same waitqueue might be woken up continuously:
>>>>>
>>>>> __sbq_wake_up __sbq_wake_up
>>>>> sbq_wake_ptr -> assume 0
>>>>> sbq_wake_ptr -> 0
>>>>> atomic_dec_return
>>>>> atomic_dec_return
>>>>> atomic_cmpxchg -> succeed
>>>>> atomic_cmpxchg -> failed
>>>>> return true
>>>>>
>>>>> __sbq_wake_up
>>>>> sbq_wake_ptr
>>>>> atomic_read(&sbq->wake_index) -> still 0
>>>>> sbq_index_atomic_inc -> inc to 1
>>>>> if (waitqueue_active(&ws->wait))
>>>>> if (wake_index != atomic_read(&sbq->wake_index))
>>>>> atomic_set -> reset from 1 to 0
>>>>> wake_up_nr -> wake up first waitqueue
>>>>> // continue to wake up in first waitqueue
>>>>>
>>>>> What's worse, io hung is possible in theory because wakeups might be
>>>>> missed. For example, 2 * wake_batch tags are put, while only wake_batch
>>>>> threads are worken:
>>>>>
>>>>> __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, refactor to make sure waitqueues will be woken up
>>>>> one by one,
>>>>>
>>>>> Signed-off-by: Yu Kuai <yukuai3@...wei.com>
>>>>
>>>> So as far as I can tell your patch does not completely fix this race. See
>>>> below:
>>>>
>>>>> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
>>>>> index ae4fd4de9ebe..dc2959cb188c 100644
>>>>> --- a/lib/sbitmap.c
>>>>> +++ b/lib/sbitmap.c
>>>>> @@ -574,66 +574,69 @@ void sbitmap_queue_min_shallow_depth(struct sbitmap_queue *sbq,
>>>>> }
>>>>> EXPORT_SYMBOL_GPL(sbitmap_queue_min_shallow_depth);
>>>>> -static struct sbq_wait_state *sbq_wake_ptr(struct sbitmap_queue *sbq)
>>>>> +static void sbq_update_wake_index(struct sbitmap_queue *sbq,
>>>>> + int old_wake_index)
>>>>> {
>>>>> int i, wake_index;
>>>>> -
>>>>> - if (!atomic_read(&sbq->ws_active))
>>>>> - return NULL;
>>>>> + struct sbq_wait_state *ws;
>>>>> wake_index = atomic_read(&sbq->wake_index);
>>>>> - for (i = 0; i < SBQ_WAIT_QUEUES; i++) {
>>>>> - struct sbq_wait_state *ws = &sbq->ws[wake_index];
>>>>> + if (old_wake_index != wake_index)
>>>>> + return;
>>>>> + for (i = 1; i < SBQ_WAIT_QUEUES; i++) {
>>>>> + wake_index = sbq_index_inc(wake_index);
>>>>> + ws = &sbq->ws[wake_index];
>>>>> + /* Find the next active waitqueue in round robin manner */
>>>>> if (waitqueue_active(&ws->wait)) {
>>>>> - if (wake_index != atomic_read(&sbq->wake_index))
>>>>> - atomic_set(&sbq->wake_index, wake_index);
>>>>> - return ws;
>>>>> + atomic_cmpxchg(&sbq->wake_index, old_wake_index,
>>>>> + wake_index);
>>>>> + return;
>>>>> }
>>>>> -
>>>>> - wake_index = sbq_index_inc(wake_index);
>>>>> }
>>>>> -
>>>>> - return NULL;
>>>>> }
>>>>> static bool __sbq_wake_up(struct sbitmap_queue *sbq)
>>>>> {
>>>>> struct sbq_wait_state *ws;
>>>>> unsigned int wake_batch;
>>>>> - int wait_cnt;
>>>>> + int wait_cnt, wake_index;
>>>>> - ws = sbq_wake_ptr(sbq);
>>>>> - if (!ws)
>>>>> + if (!atomic_read(&sbq->ws_active))
>>>>> return false;
>>>>> - wait_cnt = atomic_dec_return(&ws->wait_cnt);
>>>>> - if (wait_cnt <= 0) {
>>>>> - int ret;
>>>>> -
>>>>> - wake_batch = READ_ONCE(sbq->wake_batch);
>>>>> -
>>>>> - /*
>>>>> - * 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_index = atomic_read(&sbq->wake_index);
>>>>> + ws = &sbq->ws[wake_index];
>>>>> + /*
>>>>> + * This can only happen in the first wakeup when sbitmap waitqueues
>>>>> + * are no longer idle.
>>>>> + */
>>>>> + if (!waitqueue_active(&ws->wait)) {
>>>>> + sbq_update_wake_index(sbq, wake_index);
>>>>> + return true;
>>>>> + }
>>>>> - /*
>>>>> - * 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;
>>>>> - }
>>>>> + wait_cnt = atomic_dec_return(&ws->wait_cnt);
>>>>> + if (wait_cnt > 0)
>>>>> + return false;
>>>>
>>>> The following race is still possible:
>>>>
>>>> CPU1 CPU2
>>>> __sbq_wake_up __sbq_wake_up
>>>> wake_index = atomic_read(&sbq->wake_index);
>>>> wake_index = atomic_read(&sbq->wake_index);
>>>>
>>>> if (!waitqueue_active(&ws->wait)) -> not taken
>>>> if (!waitqueue_active(&ws->wait)) -> not taken
>>>> wait_cnt = atomic_dec_return(&ws->wait_cnt);
>>>> /* decremented to 0 now */
>>>> if (wait_cnt > 0) -> not taken
>>>> sbq_update_wake_index(sbq, wake_index);
>>>> if (wait_cnt < 0) -> not taken
>>>> ...
>>>> atomic_set(&ws->wait_cnt, wake_batch);
>>>> wake_up_nr(&ws->wait, wake_batch);
>>>> wait_cnt = atomic_dec_return(&ws->wait_cnt);
>>>> /*
>>>> * decremented to wake_batch - 1 but
>>>> * there are no tasks waiting anymore
>>>> * so the wakeup should have gone
>>>> * to a different waitqueue.
>>>> */
>>>>
>>>> I have an idea how to fix all these lost wakeups, I'll try to code it
>>>> whether it would look usable...
>> Hi, Jan
>>
>> Thanks for the analysis, it's right this is possible.
>>>
>>> Thinking a bit more about it your code would just need a small tweak like:
>>>
>>> wait_cnt = atomic_dec_return(&ws->wait_cnt);
>>> /*
>>> * Concurrent callers should call this function again
>>> * to wakeup a new batch on a different 'ws'.
>>> */
>>> if (wait_cnt < 0 || !waitqueue_active(&ws->wait)) {
>>> sbq_update_wake_index(sbq, wake_index);
>>> return true;
>>> }
>>
>> I'm thinking that if the wait_queue is still active, this will decrease
>> 'wait_cnt' in old waitqueue while 'wake_index' is already moved to next
>> waitqueue. This really broke the design...
>
> I agree this can happen and it is not ideal. On the other hand the wakeup
> is not really lost, just effectively delayed until we select this waitqueue
> again so it should not result in any hangs. And other ways to avoid the
> race seem more expensive to me...
Hi, Jan
Before you reviewed this version, I aready posted v2... It semms v2 is
using exactly the same logic that you suggested here 😉.
Thanks,
Kuai
>
> Honza
>
>>> if (wait_cnt > 0)
>>> return false;
>>> sbq_update_wake_index(sbq, wake_index);
>>>
>>> wake_batch = READ_ONCE(sbq->wake_batch);
>>> wake_up_nr(&ws->wait, wake_batch);
>>> /*
>>> * 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 results of the wakeup if
>>> * atomic_dec_return() has seen results of the atomic_set.
>>> */
>>> smp_mb__before_atomic();
>>> atomic_set(&ws->wait_cnt, wake_batch);
>>>
>>> Honza
>>>
>>>>> + sbq_update_wake_index(sbq, wake_index);
>>>>> + /*
>>>>> + * Concurrent callers should call this function again
>>>>> + * to wakeup a new batch on a different 'ws'.
>>>>> + */
>>>>> + if (wait_cnt < 0)
>>>>> return true;
>>>>> - }
>>>>> +
>>>>> + wake_batch = READ_ONCE(sbq->wake_batch);
>>>>> + /*
>>>>> + * 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();
>>>>> + atomic_set(&ws->wait_cnt, wake_batch);
>>>>> + wake_up_nr(&ws->wait, wake_batch);
>>>>> return false;
>>>>> }
>>>>> --
>>>>> 2.31.1
>>>>>
>>>> --
>>>> Jan Kara <jack@...e.com>
>>>> SUSE Labs, CR
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