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

[Ming Lei]

On Thu, Jun 06, 2024 at 03:21:38PM +0800, YangYang wrote:
> On 2024/6/6 11:12, Ming Lei wrote:
> > On Tue, Jun 04, 2024 at 02:12:22PM +0800, 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):
> > 
> > After 1) fails, sbitmap_deferred_clear() is called with spinlock,
> > then it is pretty easy to solve the race, such as, the following patch
> > against the revert patch.
> > 
> > 
> > diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> > index dee02a0266a6..c015ecd8e10e 100644
> > --- a/lib/sbitmap.c
> > +++ b/lib/sbitmap.c
> > @@ -63,13 +63,15 @@ static inline void update_alloc_hint_after_get(struct sbitmap *sb,
> >   static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
> >   {
> >   	unsigned long mask;
> > -	bool ret = false;
> >   	unsigned long flags;
> > +	bool ret;
> >   	spin_lock_irqsave(&map->swap_lock, flags);
> > -	if (!map->cleared)
> > +	if (!map->cleared) {
> > +		ret = !!map->word;
> 
> After atomic_long_andnot(mask, (atomic_long_t *)&map->word), map->word
> may be 0 if all requests have completed, or not 0 if some requests are
> still in flight.

setting ->word is lockless, but zeroing ->word is serialized with ->swap_lock.

> Therefore, using !!map->word to determine the
> availability of free tags is inaccurate.

The check should be changed to decide if any free bit is available in
map->word instead of !!map->word, and 'shift' need to be passed in
sbitmap_deferred_clear().

Just be curious, do you have reproducer for this issue?

Thanks,
Ming