erofs: simplify try_to_claim_pcluster()

simplify try_to_claim_pcluster() by directly using cmpxchg() here
(the retry loop caused more overhead.) Also, move the chain loop
detection in and rename it to z_erofs_try_to_claim_pcluster().

Link: https://lore.kernel.org/r/20201208095834.3133565-3-hsiangkao@redhat.comReviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <hsiangkao@redhat.com>

fs/erofs/zdata.c

@@ -292,34 +292,33 @@ static int z_erofs_attach_page(struct z_erofs_collector *clt,
 	return ret ? 0 : -EAGAIN;
 }
 
-static enum z_erofs_collectmode
-try_to_claim_pcluster(struct z_erofs_pcluster *pcl,
-		      z_erofs_next_pcluster_t *owned_head)
+static void z_erofs_try_to_claim_pcluster(struct z_erofs_collector *clt)
 {
-	/* let's claim these following types of pclusters */
-retry:
-	if (pcl->next == Z_EROFS_PCLUSTER_NIL) {
-		/* type 1, nil pcluster */
-		if (cmpxchg(&pcl->next, Z_EROFS_PCLUSTER_NIL,
-			    *owned_head) != Z_EROFS_PCLUSTER_NIL)
-			goto retry;
+	struct z_erofs_pcluster *pcl = clt->pcl;
+	z_erofs_next_pcluster_t *owned_head = &clt->owned_head;
 
+	/* type 1, nil pcluster (this pcluster doesn't belong to any chain.) */
+	if (cmpxchg(&pcl->next, Z_EROFS_PCLUSTER_NIL,
+		    *owned_head) == Z_EROFS_PCLUSTER_NIL) {
 		*owned_head = &pcl->next;
-		/* lucky, I am the followee :) */
-		return COLLECT_PRIMARY_FOLLOWED;
-	} else if (pcl->next == Z_EROFS_PCLUSTER_TAIL) {
-		/*
-		 * type 2, link to the end of a existing open chain,
-		 * be careful that its submission itself is governed
-		 * by the original owned chain.
-		 */
-		if (cmpxchg(&pcl->next, Z_EROFS_PCLUSTER_TAIL,
-			    *owned_head) != Z_EROFS_PCLUSTER_TAIL)
-			goto retry;
+		/* so we can attach this pcluster to our submission chain. */
+		clt->mode = COLLECT_PRIMARY_FOLLOWED;
+		return;
+	}
+
+	/*
+	 * type 2, link to the end of an existing open chain, be careful
+	 * that its submission is controlled by the original attached chain.
+	 */
+	if (cmpxchg(&pcl->next, Z_EROFS_PCLUSTER_TAIL,
+		    *owned_head) == Z_EROFS_PCLUSTER_TAIL) {
 		*owned_head = Z_EROFS_PCLUSTER_TAIL;
-		return COLLECT_PRIMARY_HOOKED;
+		clt->mode = COLLECT_PRIMARY_HOOKED;
+		clt->tailpcl = NULL;
+		return;
 	}
-	return COLLECT_PRIMARY;	/* :( better luck next time */
+	/* type 3, it belongs to a chain, but it isn't the end of the chain */
+	clt->mode = COLLECT_PRIMARY;
 }
 
 static int z_erofs_lookup_collection(struct z_erofs_collector *clt,
@@ -364,10 +363,8 @@ static int z_erofs_lookup_collection(struct z_erofs_collector *clt,
 	/* used to check tail merging loop due to corrupted images */
 	if (clt->owned_head == Z_EROFS_PCLUSTER_TAIL)
 		clt->tailpcl = pcl;
-	clt->mode = try_to_claim_pcluster(pcl, &clt->owned_head);
-	/* clean tailpcl if the current owned_head is Z_EROFS_PCLUSTER_TAIL */
-	if (clt->owned_head == Z_EROFS_PCLUSTER_TAIL)
-		clt->tailpcl = NULL;
+
+	z_erofs_try_to_claim_pcluster(clt);
 	clt->cl = cl;
 	return 0;
 }