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

[Ming Lei]

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;
 		goto out_unlock;
+	}
 
 	/*
 	 * First get a stable cleared mask, setting the old mask to 0.


Thanks, 
Ming