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

[Ming Lei <ming.lei@redhat.com>]

On Mon, Jun 17, 2024 at 10:45:51AM +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:
> 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

Can we avoid the above conditional compiling by the following way?

	word_mask = (~0UL) >> (BITS_PER_LONG - depth);


> +			if (!wrap && alloc_hint)
> +				word_mask &= ~((1UL << alloc_hint) - 1);

The current behavior is to always retry after moving ->cleared to
word, and we change it to retry in case of any free bits. So here
is it for avoiding dead loop by taking wrap & alloc_hint into
account? If yes, this way looks fine, but I'd suggest to document
this change.


Thanks, 
Ming