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

[Yu Kuai]

Hi,

在 2024/06/04 11:25, Ming Lei 写道:
> On Tue, Jun 4, 2024 at 11:12 AM Yang Yang <yang.yang@xxxxxxxx> 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.

Thanks,
Kuai

> Thanks,
>
>
> .