as-iosched: properly protect ioc_gone and ioc count

If we have multiple tasks freeing io contexts when as-iosched
is being unloaded, we could complete() ioc_gone twice. Fix that by
protecting ioc_gone complete() and clearing with a spinlock for
just that purpose. Doesn't matter from a performance perspective,
since it'll only enter that path when ioc_gone != NULL (when as-iosched
is being rmmod'ed).
Signed-off-by: NJens Axboe <jens.axboe@oracle.com>

block/as-iosched.c

@@ -151,6 +151,7 @@ enum arq_state {
 
 static DEFINE_PER_CPU(unsigned long, ioc_count);
 static struct completion *ioc_gone;
+static DEFINE_SPINLOCK(ioc_gone_lock);
 
 static void as_move_to_dispatch(struct as_data *ad, struct request *rq);
 static void as_antic_stop(struct as_data *ad);
@@ -164,8 +165,19 @@ static void free_as_io_context(struct as_io_context *aic)
 {
 	kfree(aic);
 	elv_ioc_count_dec(ioc_count);
-	if (ioc_gone && !elv_ioc_count_read(ioc_count))
-		complete(ioc_gone);
+	if (ioc_gone) {
+		/*
+		 * AS scheduler is exiting, grab exit lock and check
+		 * the pending io context count. If it hits zero,
+		 * complete ioc_gone and set it back to NULL.
+		 */
+		spin_lock(&ioc_gone_lock);
+		if (ioc_gone && !elv_ioc_count_read(ioc_count)) {
+			complete(ioc_gone);
+			ioc_gone = NULL;
+		}
+		spin_unlock(&ioc_gone_lock);
+	}
 }
 
 static void as_trim(struct io_context *ioc)
@@ -1493,7 +1505,7 @@ static void __exit as_exit(void)
 	/* ioc_gone's update must be visible before reading ioc_count */
 	smp_wmb();
 	if (elv_ioc_count_read(ioc_count))
-		wait_for_completion(ioc_gone);
+		wait_for_completion(&all_gone);
 	synchronize_rcu();
 }