Re: [PATCH v5] sbitmap: fix io hung due to race on sbitmap_word::cleared

[YangYang]

On 2024/7/5 18:58, Pavel Begunkov wrote:
> On 7/3/24 13:28, YangYang wrote:
> > On 2024/7/3 19:55, Pavel Begunkov wrote:
> > > On 7/3/24 03:28, Yang Yang 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.
> > > >
> > > > This patch achieves two purposes:
> > > > 1. Check on ->cleared and update on both ->cleared and ->word need to
> > > > be done atomically, and using spinlock could be the simplest solution.
> > > > So revert commit 661d4f55a794 ("sbitmap: remove swap_lock"), which
> > > > may cause potential race.
> > > >
> > > > 2. Add extra check in sbitmap_deferred_clear(), to identify whether
> > > > ->word has free bits.
> > > >
> > > > Fixes: 661d4f55a794 ("sbitmap: remove swap_lock")
> > >
> > > Is it blamed right? Considering that the revert alone doesn't fix
> > > the problem, it sounds like the 2nd step might need to be ported
> > > to kernels even without the blamed commit.
> >
> > Got it. How would you feel about removing
> > "Fixes: 661d4f55a794 ("sbitmap: remove swap_lock")" and modifying
> > the commit message as follows?
>
> We need a fixes tag, which commit caused it? Can it happen prior
> to ea86ea2cdced ("sbitmap: ammortize cost of clearing bits")?

The initial introduction of the sbitmap_deferred_clear function also
introduced a potential race condition.
So the fixes tag should be:
Fixes: ea86ea2cdced ("sbitmap: ammortize cost of clearing bits")

Thanks.

> > This patch achieves two purposes:
> > 1. Check on ->cleared and update on both ->cleared and ->word need to
> > be done atomically, and using spinlock could be the simplest solution.
> > 2. Add extra check in sbitmap_deferred_clear(), to identify whether
> > ->word has free bits.
> >
> > Thanks.