X-Git-Url: https://asedeno.scripts.mit.edu/gitweb/?a=blobdiff_plain;f=lib%2Fsbitmap.c;h=155fe38756ecfda251f26fa8616a325dddd8d455;hb=5f074f3e192f10c9fade898b9b3b8812e3d83342;hp=5b382c1244ede33c14016142ac2d7fec4d0608da;hpb=4823a031f096d6d5fd16fccd8a3dd51b05a6becf;p=linux.git

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);
 
 	/*