Re: [PATCH v4] sbitmap: fix io hung due to race on sbitmap_word::cleared

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

On 2024/6/17 10:45, 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")
> Signed-off-by: Yang Yang <yang.yang@vivo.com>
> 
> ---
> Changes from v3:
>    - Add more arguments to sbitmap_deferred_clear(), for those who
>      don't care about the return value, just pass 0
>    - Consider the situation when using sbitmap_get_shallow()
>    - Consider the situation when ->round_robin is true
>    - Modify commit message
> 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           | 45 ++++++++++++++++++++++++++++++++---------
>   2 files changed, 41 insertions(+), 9 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..9bd85a9b74b9 100644
> --- a/lib/sbitmap.c
> +++ b/lib/sbitmap.c
> @@ -60,12 +60,32 @@ 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)
> -{
> -	unsigned long mask;
> +static inline bool sbitmap_deferred_clear(struct sbitmap_word *map,
> +		unsigned int depth, unsigned int alloc_hint, bool wrap)
> +{
> +	unsigned long mask, flags, word_mask;
> +	bool ret = false;
> +
> +	spin_lock_irqsave(&map->swap_lock, flags);
> +
> +	if (!map->cleared) {
> +		if (depth > 0) {
> +#if BITS_PER_LONG == 64
> +			word_mask = U64_MAX >> (BITS_PER_LONG - depth);
> +#else
> +			word_mask = U32_MAX >> (BITS_PER_LONG - depth);
> +#endif
> +			if (!wrap && alloc_hint)
> +				word_mask &= ~((1UL << alloc_hint) - 1);
> +
> +			if ((READ_ONCE(map->word) & word_mask) == word_mask)
> +				ret = false;
> +			else
> +				ret = true;
> +		}
>   
> -	if (!READ_ONCE(map->cleared))
> -		return false;
> +		goto out_unlock;
> +	}
>   
>   	/*
>   	 * First get a stable cleared mask, setting the old mask to 0.
> @@ -77,7 +97,10 @@ 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 +108,7 @@ 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 +140,9 @@ 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 +153,7 @@ 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], 0, 0, 0);
>   
>   	sb->depth = depth;
>   	sb->map_nr = DIV_ROUND_UP(sb->depth, bits_per_word);
> @@ -179,7 +206,7 @@ static int sbitmap_find_bit_in_word(struct sbitmap_word *map,
>   					alloc_hint, wrap);
>   		if (nr != -1)
>   			break;
> -		if (!sbitmap_deferred_clear(map))
> +		if (!sbitmap_deferred_clear(map, depth, alloc_hint, wrap))
>   			break;
>   	} while (1);
>   
> @@ -496,7 +523,7 @@ unsigned long __sbitmap_queue_get_batch(struct sbitmap_queue *sbq, int nr_tags,
>   		unsigned int map_depth = __map_depth(sb, index);
>   		unsigned long val;
>   
> -		sbitmap_deferred_clear(map);
> +		sbitmap_deferred_clear(map, 0, 0, 0);
>   		val = READ_ONCE(map->word);
>   		if (val == (1UL << (map_depth - 1)) - 1)
>   			goto next;

Gentle ping.

Thanks.