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Message-ID: <aa873af0-9b14-4926-833f-2e4005d988c2@vivo.com>
Date: Thu, 30 May 2024 18:55:53 +0800
From: YangYang <yang.yang@...o.com>
To: Yu Kuai <yukuai1@...weicloud.com>, "yukuai (C)" <yukuai3@...wei.com>
Cc: Jens Axboe <axboe@...nel.dk>, Andrew Morton <akpm@...ux-foundation.org>,
Pavel Begunkov <asml.silence@...il.com>, linux-kernel@...r.kernel.org,
linux-block@...r.kernel.org
Subject: Re: [RFC PATCH] sbitmap: fix io hung due to race on
sbitmap_word::cleared
On 2024/5/30 9:49, Yu Kuai wrote:
> Hi,
>
> 在 2024/05/27 12:26, Yang Yang 写道:
>> 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
>> */
>
> This looks correct.
>
>>
>> 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.
>>
>> Fix this issue by adding a new flag swap_inprogress to indicate whether
>> the swap is ongoing.
>>
>> Fixes: 661d4f55a794 ("sbitmap: remove swap_lock")
>> Signed-off-by: Yang Yang <yang.yang@...o.com>
>> ---
>> include/linux/sbitmap.h | 5 +++++
>> lib/sbitmap.c | 22 ++++++++++++++++++++--
>> 2 files changed, 25 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
>> index d662cf136021..b88a9e4997ab 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_inprogress: set to 1 when swapping word <-> cleared
>> + */
>> + atomic_t swap_inprogress;
>> } ____cacheline_aligned_in_smp;
>> /**
>> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
>> index 1e453f825c05..d4bb258fe8b0 100644
>> --- a/lib/sbitmap.c
>> +++ b/lib/sbitmap.c
>> @@ -62,10 +62,19 @@ static inline void update_alloc_hint_after_get(struct sbitmap *sb,
>> */
>> static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
>> {
>> - unsigned long mask;
>> + unsigned long mask, flags;
>> + int zero = 0;
>> - if (!READ_ONCE(map->cleared))
>> + if (!READ_ONCE(map->cleared)) {
>> + if (atomic_read(&map->swap_inprogress))
>> + goto out_wait;
>> return false;
>> + }
>> +
>> + if (!atomic_try_cmpxchg(&map->swap_inprogress, &zero, 1))
>> + goto out_wait;
>> +
>> + local_irq_save(flags);
>> /*
>> * First get a stable cleared mask, setting the old mask to 0.
>> @@ -77,6 +86,15 @@ 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));
>> +
>> + atomic_set(&map->swap_inprogress, 0);
>> + smp_mb__after_atomic();
>> + local_irq_restore(flags);
>> + return true;
>> +
>> +out_wait:
>> + while (atomic_read(&map->swap_inprogress))
>> + ;
>
> However, I really don't like this change.
>
> How about following patch? I think it can fix the prblem as well.
The following patch cannot fix the problem.
Thanks.
>
> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> index 1e453f825c05..d1b9bd61d296 100644
> --- a/lib/sbitmap.c
> +++ b/lib/sbitmap.c
> @@ -175,12 +175,14 @@ static int sbitmap_find_bit_in_word(struct sbitmap_word *map,
> int nr;
>
> do {
> + bool clear = READ_ONCE(map->cleared) != 0;
> +
> nr = __sbitmap_get_word(&map->word, depth,
> alloc_hint, wrap);
> - if (nr != -1)
> - break;
> - if (!sbitmap_deferred_clear(map))
> + if (nr != -1 || !clear)
> break;
> +
> + sbitmap_deferred_clear(map);
> } while (1);
>
> return nr;
>
> Thanks,
> Kuai
>> return true;
>> }
>>
>
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