linux-kernel - Re: [PATCH v2] sbitmap: fix io hung due to race on sbitmap

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Date: Tue, 11 Jun 2024 20:53:50 +0800
From: YangYang <yang.yang@...o.com>
To: Pavel Begunkov <asml.silence@...il.com>
Cc: Jens Axboe <axboe@...nel.dk>, Andrew Morton <akpm@...ux-foundation.org>,
 linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
 "yukuai (C)" <yukuai3@...wei.com>, Yu Kuai <yukuai1@...weicloud.com>,
 Ming Lei <ming.lei@...hat.com>
Subject: Re: [PATCH v2] sbitmap: fix io hung due to race on
 sbitmap_word::cleared

On 2024/6/7 20:59, Pavel Begunkov wrote:
> On 6/4/24 08:03, YangYang wrote:
>> On 2024/6/4 14:12, Yu Kuai wrote:
>>> Hi,
>>>
>>> 在 2024/06/04 11:25, Ming Lei 写道:
>>>> On Tue, Jun 4, 2024 at 11:12 AM Yang Yang <yang.yang@...o.com> wrote:
>>>>>
>>>>> Configuration for sbq:
>>>>>    depth=64, wake_batch=6, shift=6, map_nr=1
>>>>>
>>>>> 1. There are 64 requests in progress:
>>>>>    map->word = 0xFFFFFFFFFFFFFFFF
>>>>> 2. After all the 64 requests complete, and no more requests come:
>>>>>    map->word = 0xFFFFFFFFFFFFFFFF, map->cleared = 0xFFFFFFFFFFFFFFFF
>>>>> 3. Now two tasks try to allocate requests:
>>>>>    T1:                                       T2:
>>>>>    __blk_mq_get_tag                          .
>>>>>    __sbitmap_queue_get                       .
>>>>>    sbitmap_get                               .
>>>>>    sbitmap_find_bit                          .
>>>>>    sbitmap_find_bit_in_word                  .
>>>>>    __sbitmap_get_word  -> nr=-1              __blk_mq_get_tag
>>>>>    sbitmap_deferred_clear                    __sbitmap_queue_get
>>>>>    /* map->cleared=0xFFFFFFFFFFFFFFFF */     sbitmap_find_bit
>>>>>      if (!READ_ONCE(map->cleared))           sbitmap_find_bit_in_word
>>>>>        return false;                         __sbitmap_get_word -> nr=-1
>>>>>      mask = xchg(&map->cleared, 0)           sbitmap_deferred_clear
>>>>>      atomic_long_andnot()                    /* map->cleared=0 */
>>>>>                                                if (!(map->cleared))
>>>>>                                                  return false;
>>>>>                                       /*
>>>>>                                        * map->cleared is cleared by T1
>>>>>                                        * T2 fail to acquire the tag
>>>>>                                        */
>>>>>
>>>>> 4. T2 is the sole tag waiter. When T1 puts the tag, T2 cannot be woken
>>>>> up due to the wake_batch being set at 6. If no more requests come, T1
>>>>> will wait here indefinitely.
>>>>>
>>>>> To fix this issue, simply revert commit 661d4f55a794 ("sbitmap:
>>>>> remove swap_lock"), which causes this issue.
>>>>
>>>> I'd suggest to add the following words in commit log:
>>>>
>>>> Check on ->cleared and update on both ->cleared and ->word need to be
>>>> done atomically, and using spinlock could be the simplest solution.
>>>>
>>>> Otherwise, the patch looks fine for me.
>>>
>>> Maybe I'm noob, but I'm confused how can this fix the problem, looks
>>> like the race condition doesn't change.
>>>
>>> In sbitmap_find_bit_in_word:
>>>
>>> 1) __sbitmap_get_word read word;
>>> 2) sbitmap_deferred_clear clear cleared;
>>> 3) sbitmap_deferred_clear update word;
>>>
>>> 2) and 3) are done atomically while 1) can still concurrent with 3):
>>>
>>> t1:
>>> sbitmap_find_bit_in_word
>>>   __sbitmap_get_word
>>>   -> read old word, return -1 >          t2:
>>>          sbitmap_find_bit_in_word
>>>           __sbitmap_get_word
>>>           -> read old word, return -1
>>>   sbitmap_deferred_clear
>>>   -> clear cleared and update word
>>>          sbitmap_deferred_clear
>>>          -> cleared is cleared, fail
>>
>> Yes, you are right, this patch cannot fix this issue.
> 
> One other alternative is to kill ->cleared. It's not
> immediately clear how important it is. Do we have any
> numbers?

Sorry, I can't get it. Are you suggesting to remove ->cleared from
struct sbitmap_word entirely?

Thanks.

> 
>>> BYW, I still think it's fine to fix this problem by trying the
>>> __sbitmap_get_word() at least one more time if __sbitmap_get_word()
>>> failed.
>>
>> Err, after trying one more time __sbitmap_get_word() may still fail.
>