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

[YangYang]

On 2024/6/4 9:23, Ming Lei wrote:
> On Mon, May 27, 2024 at 12:26:50PM +0800, 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.
> >
> > Fix this issue by adding a new flag swap_inprogress to indicate whether
> > the swap is ongoing.
> >
> > Fixes: 661d4f55a794 ("sbitmap: remove swap_lock")
>
> Indeed, commit 661d4f55a794 ("sbitmap: remove swap_lock") causes this
> issue, and ->cleared and ->word should have been checked & updated
> atomically.
>
> > 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))
> > +		;
> >   	return true;
> >   }
>
> IMO, the above fix looks a bit complicated, and I'd suggest to revert
> 661d4f55a794 ("sbitmap: remove swap_lock") if no clean & simple solution
> can be figured out.

Thanks. I will send a patch to revert that change.

> Thanks,
> Ming