Re: [PATCH v3] sbitmap: fix io hung due to race on sbitmap_word::cleared

[Ming Lei]

On Thu, Jun 06, 2024 at 09:57:21PM +0800, 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:
> First:
> 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.
> 
> Second:
> Add extra check in sbitmap_deferred_clear(), to identify whether
> map->cleared is cleared by another task after failing to get a tag.
> 
> Fixes: 661d4f55a794 ("sbitmap: remove swap_lock")
> Signed-off-by: Yang Yang <yang.yang@xxxxxxxx>
> 
> ---
> Changes from v2:
>   - Modify commit message by suggestion
>   - Add extra check in sbitmap_deferred_clear() by suggestion
> Changes from v1:
>   - simply revert commit 661d4f55a794 ("sbitmap: remove swap_lock")
> ---
>  include/linux/sbitmap.h |  5 +++++
>  lib/sbitmap.c           | 28 +++++++++++++++++++++-------
>  2 files changed, 26 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
> index d662cf136021..ec0b0e73c906 100644
> --- a/include/linux/sbitmap.h
> +++ b/include/linux/sbitmap.h
> @@ -36,6 +36,11 @@ struct sbitmap_word {
>  	 * @cleared: word holding cleared bits
>  	 */
>  	unsigned long cleared ____cacheline_aligned_in_smp;
> +
> +	/**
> +	 * @swap_lock: Held while swapping word <-> cleared
> +	 */
> +	spinlock_t swap_lock;
>  } ____cacheline_aligned_in_smp;
>  
>  /**
> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> index 1e453f825c05..06b837311e03 100644
> --- a/lib/sbitmap.c
> +++ b/lib/sbitmap.c
> @@ -60,12 +60,19 @@ static inline void update_alloc_hint_after_get(struct sbitmap *sb,
>  /*
>   * See if we have deferred clears that we can batch move
>   */
> -static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
> +static inline bool sbitmap_deferred_clear(struct sbitmap_word *map,
> +		unsigned int depth)
>  {
>  	unsigned long mask;
> +	unsigned long flags;
> +	bool ret;
>  
> -	if (!READ_ONCE(map->cleared))
> -		return false;
> +	spin_lock_irqsave(&map->swap_lock, flags);
> +
> +	if (!map->cleared) {
> +		ret = find_first_zero_bit(&map->word, depth) >= depth ? false : true;
> +		goto out_unlock;
> +	}

Direct check over map->word should be more efficient than find_first_zero_bit():

  		if (READ_ONCE(map->word) == (1UL << depth) - 1)
			ret = false;
		else
			ret = true;

>  
>  	/*
>  	 * First get a stable cleared mask, setting the old mask to 0.
> @@ -77,7 +84,10 @@ static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
>  	 */
>  	atomic_long_andnot(mask, (atomic_long_t *)&map->word);
>  	BUILD_BUG_ON(sizeof(atomic_long_t) != sizeof(map->word));
> -	return true;
> +	ret = true;
> +out_unlock:
> +	spin_unlock_irqrestore(&map->swap_lock, flags);
> +	return ret;
>  }
>  
>  int sbitmap_init_node(struct sbitmap *sb, unsigned int depth, int shift,
> @@ -85,6 +95,7 @@ int sbitmap_init_node(struct sbitmap *sb, unsigned int depth, int shift,
>  		      bool alloc_hint)
>  {
>  	unsigned int bits_per_word;
> +	int i;
>  
>  	if (shift < 0)
>  		shift = sbitmap_calculate_shift(depth);
> @@ -116,6 +127,9 @@ int sbitmap_init_node(struct sbitmap *sb, unsigned int depth, int shift,
>  		return -ENOMEM;
>  	}
>  
> +	for (i = 0; i < sb->map_nr; i++)
> +		spin_lock_init(&sb->map[i].swap_lock);
> +
>  	return 0;
>  }
>  EXPORT_SYMBOL_GPL(sbitmap_init_node);
> @@ -126,7 +140,7 @@ void sbitmap_resize(struct sbitmap *sb, unsigned int depth)
>  	unsigned int i;
>  
>  	for (i = 0; i < sb->map_nr; i++)
> -		sbitmap_deferred_clear(&sb->map[i]);
> +		sbitmap_deferred_clear(&sb->map[i], depth);

The above 'depth' is whole sbitmap depth, and you need to figure out
word depth from __map_depth().


Thanks, 
Ming