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

YangYang
2024-06-06T07:21:48Z

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. Therefore, using !!map->word to determine the
availability of free tags is inaccurate.

Thanks

> goto out_unlock;
> + }
>
> /*
> * First get a stable cleared mask, setting the old mask to 0.
>
>
> Thanks,
> Ming
>