target: Fix target_core_register_fabric() for built-in fabric modules

If we try to create a fabric directory in configfs for one of the
default hard-coded fabric modules (iscsi and loopback), and that
fabric is actually built into the kernel, then the operation will
spuriously fail because request_module() (for the code that's already
linked into the kernel) fails.

Fix this by running the autoprobing code only if an initial
target_core_get_fabric() fails.

Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/target_core_configfs.c

@@ -126,48 +126,57 @@ static struct config_group *target_core_register_fabric(
 
 	pr_debug("Target_Core_ConfigFS: REGISTER -> group: %p name:"
 			" %s\n", group, name);
-	/*
-	 * Below are some hardcoded request_module() calls to automatically
-	 * local fabric modules when the following is called:
-	 *
-	 * mkdir -p /sys/kernel/config/target/$MODULE_NAME
-	 *
-	 * Note that this does not limit which TCM fabric module can be
-	 * registered, but simply provids auto loading logic for modules with
-	 * mkdir(2) system calls with known TCM fabric modules.
-	 */
-	if (!strncmp(name, "iscsi", 5)) {
+
+	tf = target_core_get_fabric(name);
+	if (!tf) {
+		pr_err("target_core_register_fabric() trying autoload for %s\n",
+			name);
+
 		/*
-		 * Automatically load the LIO Target fabric module when the
-		 * following is called:
+		 * Below are some hardcoded request_module() calls to automatically
+		 * local fabric modules when the following is called:
 		 *
-		 * mkdir -p $CONFIGFS/target/iscsi
-		 */
-		ret = request_module("iscsi_target_mod");
-		if (ret < 0) {
-			pr_err("request_module() failed for"
-				" iscsi_target_mod.ko: %d\n", ret);
-			return ERR_PTR(-EINVAL);
-		}
-	} else if (!strncmp(name, "loopback", 8)) {
-		/*
-		 * Automatically load the tcm_loop fabric module when the
-		 * following is called:
+		 * mkdir -p /sys/kernel/config/target/$MODULE_NAME
 		 *
-		 * mkdir -p $CONFIGFS/target/loopback
+		 * Note that this does not limit which TCM fabric module can be
+		 * registered, but simply provids auto loading logic for modules with
+		 * mkdir(2) system calls with known TCM fabric modules.
 		 */
-		ret = request_module("tcm_loop");
-		if (ret < 0) {
-			pr_err("request_module() failed for"
-				" tcm_loop.ko: %d\n", ret);
-			return ERR_PTR(-EINVAL);
+
+		if (!strncmp(name, "iscsi", 5)) {
+			/*
+			 * Automatically load the LIO Target fabric module when the
+			 * following is called:
+			 *
+			 * mkdir -p $CONFIGFS/target/iscsi
+			 */
+			ret = request_module("iscsi_target_mod");
+			if (ret < 0) {
+				pr_err("request_module() failed for"
+				       " iscsi_target_mod.ko: %d\n", ret);
+				return ERR_PTR(-EINVAL);
+			}
+		} else if (!strncmp(name, "loopback", 8)) {
+			/*
+			 * Automatically load the tcm_loop fabric module when the
+			 * following is called:
+			 *
+			 * mkdir -p $CONFIGFS/target/loopback
+			 */
+			ret = request_module("tcm_loop");
+			if (ret < 0) {
+				pr_err("request_module() failed for"
+				       " tcm_loop.ko: %d\n", ret);
+				return ERR_PTR(-EINVAL);
+			}
 		}
+
+		tf = target_core_get_fabric(name);
 	}
 
-	tf = target_core_get_fabric(name);
 	if (!tf) {
 		pr_err("target_core_get_fabric() failed for %s\n",
-			name);
+		       name);
 		return ERR_PTR(-EINVAL);
 	}
 	pr_debug("Target_Core_ConfigFS: REGISTER -> Located fabric:"