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

[YangYang]

On 2024/5/30 9:49, Yu Kuai wrote:
> Hi,
>
> 在 2024/05/27 12:26, Yang Yang 写道:
> > 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
> >                                        */
>
> This looks correct.
>
> > 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.
> >
> > Fix this issue by adding a new flag swap_inprogress to indicate whether
> > the swap is ongoing.
> >
> > Fixes: 661d4f55a794 ("sbitmap: remove swap_lock")
> > Signed-off-by: Yang Yang <yang.yang@xxxxxxxx>
> > ---
> >   include/linux/sbitmap.h |  5 +++++
> >   lib/sbitmap.c           | 22 ++++++++++++++++++++--
> >   2 files changed, 25 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
> > index d662cf136021..b88a9e4997ab 100644
> > --- a/include/linux/sbitmap.h
> > +++ b/include/linux/sbitmap.h
> > @@ -36,6 +36,11 @@ struct sbitmap_word {
> >        * @cleared: word holding cleared bits
> >        */
> >       unsigned long cleared ____cacheline_aligned_in_smp;
> > +
> > +    /**
> > +     * @swap_inprogress: set to 1 when swapping word <-> cleared
> > +     */
> > +    atomic_t swap_inprogress;
> >   } ____cacheline_aligned_in_smp;
> >   /**
> > diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> > index 1e453f825c05..d4bb258fe8b0 100644
> > --- a/lib/sbitmap.c
> > +++ b/lib/sbitmap.c
> > @@ -62,10 +62,19 @@ static inline void update_alloc_hint_after_get(struct sbitmap *sb,
> >    */
> >   static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
> >   {
> > -    unsigned long mask;
> > +    unsigned long mask, flags;
> > +    int zero = 0;
> > -    if (!READ_ONCE(map->cleared))
> > +    if (!READ_ONCE(map->cleared)) {
> > +        if (atomic_read(&map->swap_inprogress))
> > +            goto out_wait;
> >           return false;
> > +    }
> > +
> > +    if (!atomic_try_cmpxchg(&map->swap_inprogress, &zero, 1))
> > +        goto out_wait;
> > +
> > +    local_irq_save(flags);
> >       /*
> >        * First get a stable cleared mask, setting the old mask to 0.
> > @@ -77,6 +86,15 @@ static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
> >        */
> >       atomic_long_andnot(mask, (atomic_long_t *)&map->word);
> >       BUILD_BUG_ON(sizeof(atomic_long_t) != sizeof(map->word));
> > +
> > +    atomic_set(&map->swap_inprogress, 0);
> > +    smp_mb__after_atomic();
> > +    local_irq_restore(flags);
> > +    return true;
> > +
> > +out_wait:
> > +    while (atomic_read(&map->swap_inprogress))
> > +        ;
>
> However, I really don't like this change.
>
> How about following patch? I think it can fix the prblem as well.

The following patch cannot fix the problem.

Thanks.

> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> index 1e453f825c05..d1b9bd61d296 100644
> --- a/lib/sbitmap.c
> +++ b/lib/sbitmap.c
> @@ -175,12 +175,14 @@ static int sbitmap_find_bit_in_word(struct sbitmap_word *map,
>          int nr;
>
>          do {
> +               bool clear = READ_ONCE(map->cleared) != 0;
> +
>                  nr = __sbitmap_get_word(&map->word, depth,
>                                          alloc_hint, wrap);
> -               if (nr != -1)
> -                       break;
> -               if (!sbitmap_deferred_clear(map))
> +               if (nr != -1 || !clear)
>                          break;
> +
> +               sbitmap_deferred_clear(map);
>          } while (1);
>
>          return nr;
>
> Thanks,
> Kuai
> >       return true;
> >   }