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
