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Subject: Re: [PATCH v2] sbitmap: fix io hung due to race on
 sbitmap_word::cleared
To: Ming Lei <ming.lei@redhat.com>, Yang Yang <yang.yang@vivo.com>
Cc: Jens Axboe <axboe@kernel.dk>, Andrew Morton <akpm@linux-foundation.org>,
 Pavel Begunkov <asml.silence@gmail.com>, linux-kernel@vger.kernel.org,
 linux-block@vger.kernel.org, "yukuai (C)" <yukuai3@huawei.com>
References: <20240604031124.2261-1-yang.yang@vivo.com>
 <CAFj5m9KV7OJ4_KjbSkpdtfrKamoLzV6EH-mJP3=y+VvoYOzC3w@mail.gmail.com>
From: Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <aa7246f9-f7df-3054-077e-eb21c7f423ac@huaweicloud.com>
Date: Tue, 4 Jun 2024 14:12:22 +0800
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Hi,

在 2024/06/04 11:25, Ming Lei 写道:
> On Tue, Jun 4, 2024 at 11:12 AM Yang Yang <yang.yang@vivo.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

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.

Thanks,
Kuai

> 
> Thanks,
> 
> 
> .
>