1 files changed, 11 insertions, 0 deletions

diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index 5b382c1244ed..155fe38756ec 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -591,6 +591,17 @@ EXPORT_SYMBOL_GPL(sbitmap_queue_wake_up);
 void sbitmap_queue_clear(struct sbitmap_queue *sbq, unsigned int nr,
                          unsigned int cpu)
 {
+        /*
+         * Once the clear bit is set, the bit may be allocated out.
+         *
+         * Orders READ/WRITE on the asssociated instance(such as request
+         * of blk_mq) by this bit for avoiding race with re-allocation,
+         * and its pair is the memory barrier implied in __sbitmap_get_word.
+         *
+         * One invariant is that the clear bit has to be zero when the bit
+         * is in use.
+         */
+        smp_mb__before_atomic();
         sbitmap_deferred_clear_bit(&sbq->sb, nr);
 
         /*