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

[Pavel Begunkov]

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.

--
Pavel Begunkov