sbitmap: fix io hung due to race on sbitmap_word::cleared

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.

This patch achieves two purposes:
1. Check on ->cleared and update on both ->cleared and ->word need to
be done atomically, and using spinlock could be the simplest solution.
2. Add extra check in sbitmap_deferred_clear(), to identify whether
->word has free bits.

Fixes: ea86ea2cdced ("sbitmap: ammortize cost of clearing bits")
Signed-off-by: Yang Yang <yang.yang@vivo.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20240716082644.659566-1-yang.yang@vivo.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
index d662cf136021d6b95f9e8a36081fea95a3b6527a..c09cdcc99471e170433e745f640b0a4fffbb0833 100644 (file)
--- 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: serializes simultaneous updates of ->word and ->cleared
+        */
+       spinlock_t swap_lock;
 } ____cacheline_aligned_in_smp;
 
 /**

diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index 1e453f825c05d34f299abcecb12a94c2fe9d023b..5e2e93307f0d05f94ee9bde92144ded6f6c733d0 100644 (file)
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -60,12 +60,30 @@ static inline void update_alloc_hint_after_get(struct sbitmap *sb,
 /*
  * See if we have deferred clears that we can batch move
  */
-static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
+static inline bool sbitmap_deferred_clear(struct sbitmap_word *map,
+               unsigned int depth, unsigned int alloc_hint, bool wrap)
 {
-       unsigned long mask;
+       unsigned long mask, word_mask;
 
-       if (!READ_ONCE(map->cleared))
-               return false;
+       guard(spinlock_irqsave)(&map->swap_lock);
+
+       if (!map->cleared) {
+               if (depth == 0)
+                       return false;
+
+               word_mask = (~0UL) >> (BITS_PER_LONG - depth);
+               /*
+                * The current behavior is to always retry after moving
+                * ->cleared to word, and we change it to retry in case
+                * of any free bits. To avoid an infinite loop, we need
+                * to take wrap & alloc_hint into account, otherwise a
+                * soft lockup may occur.
+                */
+               if (!wrap && alloc_hint)
+                       word_mask &= ~((1UL << alloc_hint) - 1);
+
+               return (READ_ONCE(map->word) & word_mask) != word_mask;
+       }
 
        /*
         * First get a stable cleared mask, setting the old mask to 0.
@@ -85,6 +103,7 @@ 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 +135,9 @@ 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);
@@ -126,7 +148,7 @@ void sbitmap_resize(struct sbitmap *sb, unsigned int depth)
        unsigned int i;
 
        for (i = 0; i < sb->map_nr; i++)
-               sbitmap_deferred_clear(&sb->map[i]);
+               sbitmap_deferred_clear(&sb->map[i], 0, 0, 0);
 
        sb->depth = depth;
        sb->map_nr = DIV_ROUND_UP(sb->depth, bits_per_word);
@@ -179,7 +201,7 @@ static int sbitmap_find_bit_in_word(struct sbitmap_word *map,
                                        alloc_hint, wrap);
                if (nr != -1)
                        break;
-               if (!sbitmap_deferred_clear(map))
+               if (!sbitmap_deferred_clear(map, depth, alloc_hint, wrap))
                        break;
        } while (1);
 
@@ -496,7 +518,7 @@ unsigned long __sbitmap_queue_get_batch(struct sbitmap_queue *sbq, int nr_tags,
                unsigned int map_depth = __map_depth(sb, index);
                unsigned long val;
 
-               sbitmap_deferred_clear(map);
+               sbitmap_deferred_clear(map, 0, 0, 0);
                val = READ_ONCE(map->word);
                if (val == (1UL << (map_depth - 1)) - 1)
                        goto next;