[RFC PATCH] 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.

Fix this issue by adding a new flag swap_inprogress to indicate whether
the swap is ongoing.

Fixes: 661d4f55a794 ("sbitmap: remove swap_lock")
Signed-off-by: Yang Yang <yang.yang@xxxxxxxx>
---
 include/linux/sbitmap.h |  5 +++++
 lib/sbitmap.c           | 22 ++++++++++++++++++++--
 2 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
index d662cf136021..b88a9e4997ab 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_inprogress: set to 1 when swapping word <-> cleared
+	 */
+	atomic_t swap_inprogress;
 } ____cacheline_aligned_in_smp;
 
 /**
diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index 1e453f825c05..d4bb258fe8b0 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -62,10 +62,19 @@ static inline void update_alloc_hint_after_get(struct sbitmap *sb,
  */
 static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
 {
-	unsigned long mask;
+	unsigned long mask, flags;
+	int zero = 0;
 
-	if (!READ_ONCE(map->cleared))
+	if (!READ_ONCE(map->cleared)) {
+		if (atomic_read(&map->swap_inprogress))
+			goto out_wait;
 		return false;
+	}
+
+	if (!atomic_try_cmpxchg(&map->swap_inprogress, &zero, 1))
+		goto out_wait;
+
+	local_irq_save(flags);
 
 	/*
 	 * First get a stable cleared mask, setting the old mask to 0.
@@ -77,6 +86,15 @@ 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));
+
+	atomic_set(&map->swap_inprogress, 0);
+	smp_mb__after_atomic();
+	local_irq_restore(flags);
+	return true;
+
+out_wait:
+	while (atomic_read(&map->swap_inprogress))
+		;
 	return true;
 }
 
-- 
2.34.1