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Date: Mon, 1 Jul 2024 14:39:27 +0800
From: YangYang <yang.yang@...o.com>
To: Andrew Morton <akpm@...ux-foundation.org>, Jens Axboe <axboe@...nel.dk>,
Pavel Begunkov <asml.silence@...il.com>, linux-kernel@...r.kernel.org,
linux-block@...r.kernel.org
Cc: Ming Lei <ming.lei@...hat.com>
Subject: Re: [PATCH v4] sbitmap: fix io hung due to race on
sbitmap_word::cleared
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@...o.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.
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