dm cache policy smq: handle races with queuing background_work

The background_tracker holds a set of promotions/demotions that the
cache policy wishes the core target to implement.

When adding a new operation to the tracker it's possible that an
operation on the same block is already present (but in practise this
doesn't appear to be happening).  Catch these situations and do the
appropriate cleanup.
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-cache-policy-smq.c

@@ -1174,12 +1174,16 @@ static void queue_writeback(struct smq_policy *mq)
 		work.cblock = infer_cblock(mq, e);
 
 		r = btracker_queue(mq->bg_work, &work, NULL);
-		WARN_ON_ONCE(r); // FIXME: finish, I think we have to get rid of this race.
+		if (r) {
+			clear_pending(mq, e);
+			q_push_front(&mq->dirty, e);
+		}
 	}
 }
 
 static void queue_demotion(struct smq_policy *mq)
 {
+	int r;
 	struct policy_work work;
 	struct entry *e;
 
@@ -1199,12 +1203,17 @@ static void queue_demotion(struct smq_policy *mq)
 	work.op = POLICY_DEMOTE;
 	work.oblock = e->oblock;
 	work.cblock = infer_cblock(mq, e);
-	btracker_queue(mq->bg_work, &work, NULL);
+	r = btracker_queue(mq->bg_work, &work, NULL);
+	if (r) {
+		clear_pending(mq, e);
+		q_push_front(&mq->clean, e);
+	}
 }
 
 static void queue_promotion(struct smq_policy *mq, dm_oblock_t oblock,
 			    struct policy_work **workp)
 {
+	int r;
 	struct entry *e;
 	struct policy_work work;
 
@@ -1234,7 +1243,9 @@ static void queue_promotion(struct smq_policy *mq, dm_oblock_t oblock,
 	work.op = POLICY_PROMOTE;
 	work.oblock = oblock;
 	work.cblock = infer_cblock(mq, e);
-	btracker_queue(mq->bg_work, &work, workp);
+	r = btracker_queue(mq->bg_work, &work, workp);
+	if (r)
+		free_entry(&mq->cache_alloc, e);
 }
 
 /*----------------------------------------------------------------*/