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Message-ID: <64a8dcab-4f3f-49de-9940-129307371c84@vivo.com>
Date: Tue, 11 Jun 2024 20:28:26 +0800
From: YangYang <yang.yang@...o.com>
To: Ming Lei <ming.lei@...hat.com>
Cc: 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
Subject: Re: [PATCH v3] sbitmap: fix io hung due to race on
sbitmap_word::cleared
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@...o.com>
>>
>> ---
>> 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.
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