[PATCH v7] 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:
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@xxxxxxxx>
Reviewed-by: Ming Lei <ming.lei@xxxxxxxxxx>

---
Changes from v6:
  - Use guard() for locking by suggestion
  - Modify comments by suggestion
Changes from v5:
  - Modify commit message
  - Change the fixes tag
Changes from v4:
  - Add some comments according to suggestion
Changes from v3:
  - Add more arguments to sbitmap_deferred_clear(), for those who
    don't care about the return value, just pass 0
  - Consider the situation when using sbitmap_get_shallow()
  - Consider the situation when ->round_robin is true
  - Modify commit message
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           | 41 ++++++++++++++++++++++++++++++++++-------
 2 files changed, 39 insertions(+), 7 deletions(-)

diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
index d662cf136021..c09cdcc99471 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: 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 1e453f825c05..3c00fe4b74fc 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -60,12 +60,35 @@ 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;
+	bool ret = false;
 
-	if (!READ_ONCE(map->cleared))
-		return false;
+	guard(spinlock_irqsave)(&map->swap_lock);
+
+	if (!map->cleared) {
+		if (depth > 0) {
+			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);
+
+			if ((READ_ONCE(map->word) & word_mask) == word_mask)
+				ret = false;
+			else
+				ret = true;
+		}
+
+		return ret;
+	}
 
 	/*
 	 * First get a stable cleared mask, setting the old mask to 0.
@@ -85,6 +108,7 @@ 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 +140,9 @@ 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 +153,7 @@ 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 +206,7 @@ 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 +523,7 @@ 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;
-- 
2.34.1