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

[YangYang]

On 2024/6/4 14:12, 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):
>
> t1:
> sbitmap_find_bit_in_word
>   __sbitmap_get_word
>   -> read old word, return -1
>          t2:
>          sbitmap_find_bit_in_word
>           __sbitmap_get_word
>           -> read old word, return -1
>   sbitmap_deferred_clear
>   -> clear cleared and update word
>          sbitmap_deferred_clear
>          -> cleared is cleared, fail
>
> BYW, I still think it's fine to fix this problem by trying the
> __sbitmap_get_word() at least one more time if __sbitmap_get_word()
> failed.

How about this one:
1. Add extra check in sbitmap_find_bit_in_word() referenced from
   Yu kuai's suggestion.
2. Change from atomic_long_andnot to atomic_long_fetch_andnot_release

---
 include/linux/sbitmap.h |  5 +++++
 lib/sbitmap.c           | 23 ++++++++++++++++++-----
 2 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
index d662cf136021..ec0b0e73c906 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_lock: Held while swapping word <-> cleared
+    */
+   spinlock_t swap_lock;
 } ____cacheline_aligned_in_smp;

 /**
diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index 1e453f825c05..63dadf91e40b 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -63,9 +63,13 @@ 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;

-   if (!READ_ONCE(map->cleared))
-       return false;
+   spin_lock_irqsave(&map->swap_lock, flags);
+
+   if (!map->cleared)
+       goto out_unlock;

    /*
     * First get a stable cleared mask, setting the old mask to 0.
@@ -75,9 +79,12 @@ static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
    /*
     * Now clear the masked bits in our free word
     */
-   atomic_long_andnot(mask, (atomic_long_t *)&map->word);
+   atomic_long_fetch_andnot_release(mask, (atomic_long_t *)&map->word);
    BUILD_BUG_ON(sizeof(atomic_long_t) != sizeof(map->word));
-   return true;
+   ret = true;
+out_unlock:
+   spin_unlock_irqrestore(&map->swap_lock, flags);
+   return ret;
 }

 int sbitmap_init_node(struct sbitmap *sb, unsigned int depth, int shift,
@@ -85,6 +92,7 @@ int sbitmap_init_node(struct sbitmap *sb, unsigned int depth, int shift,
              bool alloc_hint)
 {
    unsigned int bits_per_word;
+   int i;

    if (shift < 0)
        shift = sbitmap_calculate_shift(depth);
@@ -116,6 +124,9 @@ int sbitmap_init_node(struct sbitmap *sb, unsigned int depth, int shift,
        return -ENOMEM;
    }

+   for (i = 0; i < sb->map_nr; i++)
+       spin_lock_init(&sb->map[i].swap_lock);
+
    return 0;
 }
 EXPORT_SYMBOL_GPL(sbitmap_init_node);
@@ -175,11 +186,13 @@ static int sbitmap_find_bit_in_word(struct sbitmap_word *map,
    int nr;

    do {
+       unsigned long cleared = READ_ONCE(map->cleared);
+
        nr = __sbitmap_get_word(&map->word, depth,
                    alloc_hint, wrap);
        if (nr != -1)
            break;
-       if (!sbitmap_deferred_clear(map))
+       if (!sbitmap_deferred_clear(map) && !cleared)
            break;
    } while (1);

--
2.34.1

> Thanks,
> Kuai
>
> > Thanks,
> >
> >
> > .