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

[Ming Lei]

On Fri, Jun 07, 2024 at 01:59:07PM +0100, Pavel Begunkov wrote:
> 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?

Please see commit ea86ea2cdced ("sbitmap: ammortize cost of clearing bits").

```
In a threaded poll test case, half the overhead of getting and clearing
tags is removed with this change.
```

Thanks, 
Ming