linux-kernel - Re: [RFC PATCH] sbitmap: fix io hung due to race on sbitmap

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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;
>>   }
>>
>