[PATCH v3] sbitmap: fix io hung due to race on sbitmap_word::cleared

[Yang Yang]

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:
First:
Check on ->cleared and update on both ->cleared and ->word need to be
done atomically, and using spinlock could be the simplest solution.
So revert commit 661d4f55a794 ("sbitmap: remove swap_lock"), which
may cause potential race.

Second:
Add extra check in sbitmap_deferred_clear(), to identify whether
map->cleared is cleared by another task after failing to get a tag.

Fixes: 661d4f55a794 ("sbitmap: remove swap_lock")
Signed-off-by: Yang Yang <yang.yang@xxxxxxxx>

---
Changes from v2:
  - Modify commit message by suggestion
  - Add extra check in sbitmap_deferred_clear() by suggestion
Changes from v1:
  - simply revert commit 661d4f55a794 ("sbitmap: remove swap_lock")
---
 include/linux/sbitmap.h |  5 +++++
 lib/sbitmap.c           | 28 +++++++++++++++++++++-------
 2 files changed, 26 insertions(+), 7 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..06b837311e03 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -60,12 +60,19 @@ 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 long mask;
+	unsigned long flags;
+	bool ret;
 
-	if (!READ_ONCE(map->cleared))
-		return false;
+	spin_lock_irqsave(&map->swap_lock, flags);
+
+	if (!map->cleared) {
+		ret = find_first_zero_bit(&map->word, depth) >= depth ? false : true;
+		goto out_unlock;
+	}
 
 	/*
 	 * First get a stable cleared mask, setting the old mask to 0.
@@ -77,7 +84,10 @@ 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));
-	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 +95,7 @@ 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 +127,9 @@ 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 +140,7 @@ 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], depth);
 
 	sb->depth = depth;
 	sb->map_nr = DIV_ROUND_UP(sb->depth, bits_per_word);
@@ -179,7 +193,7 @@ 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))
 			break;
 	} while (1);
 
@@ -496,7 +510,7 @@ 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, map_depth);
 		val = READ_ONCE(map->word);
 		if (val == (1UL << (map_depth - 1)) - 1)
 			goto next;
-- 
2.34.1