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

[Pavel Begunkov]

On 6/4/24 08:03, 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@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
>
> Yes, you are right, this patch cannot fix this issue.

One other alternative is to kill ->cleared. It's not
immediately clear how important it is. Do we have any
numbers?


> > 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.
>
> Err, after trying one more time __sbitmap_get_word() may still fail.

--
Pavel Begunkov