linux-kernel - Re: [PATCH v5] sbitmap: fix io hung due to race on sbitmap

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Message-ID: <59c125f5-ff40-4a36-b095-b0dbf1a0a3fd@vivo.com>
Date: Wed, 3 Jul 2024 20:28:06 +0800
From: YangYang <yang.yang@...o.com>
To: Pavel Begunkov <asml.silence@...il.com>, 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 2024/7/3 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?

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.