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Date: Mon, 20 Jun 2022 21:44:16 +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 2022/06/20 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...
Thanks,
Kuai
> 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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