Re: [PATCH v2] sbitmap: fix io hung due to race on sbitmap_word::cleared

YangYang
2024-06-07T04:23:42Z

On 2024/6/6 23:48, Ming Lei wrote:
> On Thu, Jun 06, 2024 at 04:55:17PM +0800, YangYang wrote:
>> On 2024/6/4 14:12, Yu Kuai wrote:
>>> Hi,
>>>
>>> 在 2024/06/04 11:25, Ming Lei 写道:
>>>> On Tue, Jun 4, 2024 at 11:12 AM Yang Yang <yang.yang@vivo.com> 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.
>>>>>
>>>>> To fix this issue, simply revert commit 661d4f55a794 ("sbitmap:
>>>>> remove swap_lock"), which causes this issue.
>>>>
>>>> I'd suggest to add the following words in commit log:
>>>>
>>>> Check on ->cleared and update on both ->cleared and ->word need to be
>>>> done atomically, and using spinlock could be the simplest solution.
>>>>
>>>> Otherwise, the patch looks fine for me.
>>>
>>> Maybe I'm noob, but I'm confused how can this fix the problem, looks
>>> like the race condition doesn't change.
>>>
>>> In sbitmap_find_bit_in_word:
>>>
>>> 1) __sbitmap_get_word read word;
>>> 2) sbitmap_deferred_clear clear cleared;
>>> 3) sbitmap_deferred_clear update word;
>>>
>>> 2) and 3) are done atomically while 1) can still concurrent with 3):
>>>
>>> t1:
>>> sbitmap_find_bit_in_word
>>>  __sbitmap_get_word
>>>  -> read old word, return -1
>>>         t2:
>>>         sbitmap_find_bit_in_word
>>>          __sbitmap_get_word
>>>          -> read old word, return -1
>>>  sbitmap_deferred_clear
>>>  -> clear cleared and update word
>>>         sbitmap_deferred_clear
>>>         -> cleared is cleared, fail
>>>
>>> BYW, I still think it's fine to fix this problem by trying the
>>> __sbitmap_get_word() at least one more time if __sbitmap_get_word()
>>> failed.
>>
>> How about this one:
>> 1. Add extra check in sbitmap_find_bit_in_word() referenced from
>> Yu kuai's suggestion.
>> 2. Change from atomic_long_andnot to atomic_long_fetch_andnot_release
>>
>> ---
>> include/linux/sbitmap.h | 5 +++++
>> lib/sbitmap.c | 23 ++++++++++++++++++-----
>> 2 files changed, 23 insertions(+), 5 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..63dadf91e40b 100644
>> --- a/lib/sbitmap.c
>> +++ b/lib/sbitmap.c
>> @@ -63,9 +63,13 @@ static inline void update_alloc_hint_after_get(struct sbitmap *sb,
>> static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
>> {
>> unsigned long mask;
>> + bool ret = false;
>> + unsigned long flags;
>>
>> - if (!READ_ONCE(map->cleared))
>> - return false;
>> + spin_lock_irqsave(&map->swap_lock, flags);
>> +
>> + if (!map->cleared)
>> + goto out_unlock;
>>
>> /*
>> * First get a stable cleared mask, setting the old mask to 0.
>> @@ -75,9 +79,12 @@ static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
>> /*
>> * Now clear the masked bits in our free word
>> */
>> - atomic_long_andnot(mask, (atomic_long_t *)&map->word);
>> + atomic_long_fetch_andnot_release(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 +92,7 @@ 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 +124,9 @@ 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);
>> @@ -175,11 +186,13 @@ static int sbitmap_find_bit_in_word(struct sbitmap_word *map,
>> int nr;
>>
>> do {
>> + unsigned long cleared = READ_ONCE(map->cleared);
>> +
>
> ->cleared is stored in standalone cacheline, the above line adds one extra l1
> load, so I still prefer to check ->word & ->cleared in sbitmap_deferred_clear().

Yes, it's very reasonable.
I have sent V3, which does the check in sbitmap_deferred_clear()
referenced from your suggestion.

+ if (!map->cleared) {
+ ret = find_first_zero_bit(&map->word, depth) >= depth ? false : true;
+ goto out_unlock;
+ }

https://lore.kernel.org/linux-block/20240606135723.2794-1-yang.yang@vivo.com/T/#u

Thanks.