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Date: Tue, 2 Jul 2024 15:57:00 +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 v4] sbitmap: fix io hung due to race on
sbitmap_word::cleared
On 2024/7/2 11:34, Ming Lei wrote:
> 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@...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
>
> Can we avoid the above conditional compiling by the following way?
>
> word_mask = (~0UL) >> (BITS_PER_LONG - depth);
Sure, this looks better.
>
>> + 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.
Yes. Thank you for the suggestion. I will include the following
descriptions in the next version.
/*
* The current behavior is to always retry after moving
* ->cleared to word, and we change it to retry in case
* of any free bits. To avoid dead loop, we need to take
* wrap & alloc_hint into account. Without this, a soft
* lockup was detected in our test environment.
*/
Thanks.
>
> Thanks,
> Ming
>
>
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