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

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Message-ID: <CAFj5m9KfY+0r8eR-ThS7FF+xTnLaWGVrC4vT01LUChVQMqqzUA@mail.gmail.com>
Date: Wed, 3 Jul 2024 11:30:57 +0800
From: Ming Lei <ming.lei@...hat.com>
To: Yang Yang <yang.yang@...o.com>
Cc: Jens Axboe <axboe@...nel.dk>, Andrew Morton <akpm@...ux-foundation.org>, 
	Pavel Begunkov <asml.silence@...il.com>, 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 Wed, Jul 3, 2024 at 10:28 AM Yang Yang <yang.yang@...o.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.
>
> 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")
> Signed-off-by: Yang Yang <yang.yang@...o.com>

Reviewed-by: Ming Lei <ming.lei@...hat.com>