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

YangYang
2024-06-06T09:20:28Z

On 2024/6/6 17:02, Ming Lei wrote:
> On Thu, Jun 06, 2024 at 03:21:38PM +0800, YangYang wrote:
>> On 2024/6/6 11:12, Ming Lei wrote:
>>> On Tue, Jun 04, 2024 at 02:12:22PM +0800, 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):
>>>
>>> After 1) fails, sbitmap_deferred_clear() is called with spinlock,
>>> then it is pretty easy to solve the race, such as, the following patch
>>> against the revert patch.
>>>
>>>
>>> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
>>> index dee02a0266a6..c015ecd8e10e 100644
>>> --- a/lib/sbitmap.c
>>> +++ b/lib/sbitmap.c
>>> @@ -63,13 +63,15 @@ 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;
>>> + bool ret;
>>> spin_lock_irqsave(&map->swap_lock, flags);
>>> - if (!map->cleared)
>>> + if (!map->cleared) {
>>> + ret = !!map->word;
>>
>> After atomic_long_andnot(mask, (atomic_long_t *)&map->word), map->word
>> may be 0 if all requests have completed, or not 0 if some requests are
>> still in flight.
>
> setting ->word is lockless, but zeroing ->word is serialized with ->swap_lock.
>
>> Therefore, using !!map->word to determine the
>> availability of free tags is inaccurate.
>
> The check should be changed to decide if any free bit is available in
> map->word instead of !!map->word, and 'shift' need to be passed in
> sbitmap_deferred_clear().
>
> Just be curious, do you have reproducer for this issue?

It's hard to reproduce, but it's reproducible. We've encountered this
issue several times.

Thanks

>
> Thanks,
> Ming
>