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

[YangYang]

On 2024/6/7 17:44, Ming Lei wrote:
> 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;

Got it. A little modified as follows, because:
1. The depth may be shallow_depth, which is less than __map_depth()
2. (1UL << depth) may overflow

 72 #if BITS_PER_LONG == 64
 73         unsigned long word_mask = U64_MAX >> (BITS_PER_LONG - depth);
 74 #else
 75         unsigned long word_mask = U32_MAX >> (BITS_PER_LONG - depth);
 76 #endif
 77
 78         if ((READ_ONCE(map->word) & word_mask) == word_mask)
 79             ret = false;
 80         else
 81             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().

Got it.

Thanks.