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

[Pavel Begunkov <asml.silence@gmail.com>]

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")?


> 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.

-- 
Pavel Begunkov