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Message-ID: <6379eb83-0ab7-458d-af83-5d2ed1d8fec5@vivo.com>
Date: Mon, 8 Jul 2024 15:54:35 +0800
From: YangYang <yang.yang@...o.com>
To: Pavel Begunkov <asml.silence@...il.com>
Cc: Ming Lei <ming.lei@...hat.com>, Jens Axboe <axboe@...nel.dk>,
Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org,
linux-block@...r.kernel.org
Subject: Re: [PATCH v5] sbitmap: fix io hung due to race on
sbitmap_word::cleared
On 2024/7/5 18:58, Pavel Begunkov wrote:
> On 7/3/24 13:28, YangYang wrote:
>> On 2024/7/3 19:55, Pavel Begunkov wrote:
>>> On 7/3/24 03:28, Yang Yang 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.
>>>>
>>>> This patch achieves two purposes:
>>>> 1. Check on ->cleared and update on both ->cleared and ->word need to
>>>> be done atomically, and using spinlock could be the simplest solution.
>>>> So revert commit 661d4f55a794 ("sbitmap: remove swap_lock"), which
>>>> may cause potential race.
>>>>
>>>> 2. Add extra check in sbitmap_deferred_clear(), to identify whether
>>>> ->word has free bits.
>>>>
>>>> Fixes: 661d4f55a794 ("sbitmap: remove swap_lock")
>>>
>>> Is it blamed right? Considering that the revert alone doesn't fix
>>> the problem, it sounds like the 2nd step might need to be ported
>>> to kernels even without the blamed commit.
>>>
>>
>> Got it. How would you feel about removing
>> "Fixes: 661d4f55a794 ("sbitmap: remove swap_lock")" and modifying
>> the commit message as follows?
>
> We need a fixes tag, which commit caused it? Can it happen prior
> to ea86ea2cdced ("sbitmap: ammortize cost of clearing bits")?
The initial introduction of the sbitmap_deferred_clear function also
introduced a potential race condition.
So the fixes tag should be:
Fixes: ea86ea2cdced ("sbitmap: ammortize cost of clearing bits")
Thanks.
>
>> This patch achieves two purposes:
>> 1. Check on ->cleared and update on both ->cleared and ->word need to
>> be done atomically, and using spinlock could be the simplest solution.
>> 2. Add extra check in sbitmap_deferred_clear(), to identify whether
>> ->word has free bits.
>>
>> Thanks.
>
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