cifs: do all necessary checks for credits within or before locking

All the server credits and in-flight info is protected by req_lock.
Once the req_lock is held, and we've determined that we have enough
credits to continue, this lock cannot be dropped till we've made the
changes to credits and in-flight count.

However, we used to drop the lock in order to avoid deadlock with
the recent srv_lock. This could cause the checks already made to be
invalidated.

Fixed it by moving the server status check to before locking req_lock.

Fixes: d7d7a66a ("cifs: avoid use of global locks for high contention data")
Signed-off-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/smb/client/smb2ops.c

@@ -215,6 +215,16 @@ smb2_wait_mtu_credits(struct TCP_Server_Info *server, unsigned int size,
 
 	spin_lock(&server->req_lock);
 	while (1) {
+		spin_unlock(&server->req_lock);
+
+		spin_lock(&server->srv_lock);
+		if (server->tcpStatus == CifsExiting) {
+			spin_unlock(&server->srv_lock);
+			return -ENOENT;
+		}
+		spin_unlock(&server->srv_lock);
+
+		spin_lock(&server->req_lock);
 		if (server->credits <= 0) {
 			spin_unlock(&server->req_lock);
 			cifs_num_waiters_inc(server);
@@ -225,15 +235,6 @@ smb2_wait_mtu_credits(struct TCP_Server_Info *server, unsigned int size,
 				return rc;
 			spin_lock(&server->req_lock);
 		} else {
-			spin_unlock(&server->req_lock);
-			spin_lock(&server->srv_lock);
-			if (server->tcpStatus == CifsExiting) {
-				spin_unlock(&server->srv_lock);
-				return -ENOENT;
-			}
-			spin_unlock(&server->srv_lock);
-
-			spin_lock(&server->req_lock);
 			scredits = server->credits;
 			/* can deadlock with reopen */
 			if (scredits <= 8) {

fs/smb/client/transport.c

@@ -522,6 +522,16 @@ wait_for_free_credits(struct TCP_Server_Info *server, const int num_credits,
 	}
 
 	while (1) {
+		spin_unlock(&server->req_lock);
+
+		spin_lock(&server->srv_lock);
+		if (server->tcpStatus == CifsExiting) {
+			spin_unlock(&server->srv_lock);
+			return -ENOENT;
+		}
+		spin_unlock(&server->srv_lock);
+
+		spin_lock(&server->req_lock);
 		if (*credits < num_credits) {
 			scredits = *credits;
 			spin_unlock(&server->req_lock);
@@ -547,15 +557,6 @@ wait_for_free_credits(struct TCP_Server_Info *server, const int num_credits,
 				return -ERESTARTSYS;
 			spin_lock(&server->req_lock);
 		} else {
-			spin_unlock(&server->req_lock);
-
-			spin_lock(&server->srv_lock);
-			if (server->tcpStatus == CifsExiting) {
-				spin_unlock(&server->srv_lock);
-				return -ENOENT;
-			}
-			spin_unlock(&server->srv_lock);
-
 			/*
 			 * For normal commands, reserve the last MAX_COMPOUND
 			 * credits to compound requests.
@@ -569,7 +570,6 @@ wait_for_free_credits(struct TCP_Server_Info *server, const int num_credits,
 			 * for servers that are slow to hand out credits on
 			 * new sessions.
 			 */
-			spin_lock(&server->req_lock);
 			if (!optype && num_credits == 1 &&
 			    server->in_flight > 2 * MAX_COMPOUND &&
 			    *credits <= MAX_COMPOUND) {