Re: [PATCH v5] sbitmap: fix io hung due to race on sbitmap_word::cleared

[YangYang <yang.yang@vivo.com>]

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.
>