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Message-ID: <e1cdf579-007b-415f-9e4d-3fadd6f97b36@vivo.com>
Date: Tue, 4 Jun 2024 15:03:36 +0800
From: YangYang <yang.yang@...o.com>
To: Yu Kuai <yukuai1@...weicloud.com>, Ming Lei <ming.lei@...hat.com>
Cc: Jens Axboe <axboe@...nel.dk>, Andrew Morton <akpm@...ux-foundation.org>,
Pavel Begunkov <asml.silence@...il.com>, linux-kernel@...r.kernel.org,
linux-block@...r.kernel.org, "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH v2] sbitmap: fix io hung due to race on
sbitmap_word::cleared
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.
>
> 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.
>
> Thanks,
> Kuai
>
>>
>> Thanks,
>>
>>
>> .
>>
>
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