CIFS: Fix credit computation for compounded requests

In SMB3 protocol every part of the compound chain consumes credits
individually, so we need to call wait_for_free_credits() for each
of the PDUs in the chain. If an operation is interrupted, we must
ensure we return all credits taken from the server structure back.

Without this patch server can sometimes disconnect the session
due to credit mismatches, especially when first operation(s)
are large writes.
Signed-off-by: Pavel Shilovsky <pshilov@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
CC: Stable <stable@vger.kernel.org>

fs/cifs/transport.c

@@ -795,7 +795,8 @@ compound_send_recv(const unsigned int xid, struct cifs_ses *ses,
 	int i, j, rc = 0;
 	int timeout, optype;
 	struct mid_q_entry *midQ[MAX_COMPOUND];
-	unsigned int credits = 0;
+	bool cancelled_mid[MAX_COMPOUND] = {false};
+	unsigned int credits[MAX_COMPOUND] = {0};
 	char *buf;
 
 	timeout = flags & CIFS_TIMEOUT_MASK;
@@ -813,13 +814,31 @@ compound_send_recv(const unsigned int xid, struct cifs_ses *ses,
 		return -ENOENT;
 
 	/*
-	 * Ensure that we do not send more than 50 overlapping requests
-	 * to the same server. We may make this configurable later or
-	 * use ses->maxReq.
+	 * Ensure we obtain 1 credit per request in the compound chain.
+	 * It can be optimized further by waiting for all the credits
+	 * at once but this can wait long enough if we don't have enough
+	 * credits due to some heavy operations in progress or the server
+	 * not granting us much, so a fallback to the current approach is
+	 * needed anyway.
 	 */
-	rc = wait_for_free_request(ses->server, timeout, optype);
-	if (rc)
-		return rc;
+	for (i = 0; i < num_rqst; i++) {
+		rc = wait_for_free_request(ses->server, timeout, optype);
+		if (rc) {
+			/*
+			 * We haven't sent an SMB packet to the server yet but
+			 * we already obtained credits for i requests in the
+			 * compound chain - need to return those credits back
+			 * for future use. Note that we need to call add_credits
+			 * multiple times to match the way we obtained credits
+			 * in the first place and to account for in flight
+			 * requests correctly.
+			 */
+			for (j = 0; j < i; j++)
+				add_credits(ses->server, 1, optype);
+			return rc;
+		}
+		credits[i] = 1;
+	}
 
 	/*
 	 * Make sure that we sign in the same order that we send on this socket
@@ -835,8 +854,10 @@ compound_send_recv(const unsigned int xid, struct cifs_ses *ses,
 			for (j = 0; j < i; j++)
 				cifs_delete_mid(midQ[j]);
 			mutex_unlock(&ses->server->srv_mutex);
+
 			/* Update # of requests on wire to server */
-			add_credits(ses->server, 1, optype);
+			for (j = 0; j < num_rqst; j++)
+				add_credits(ses->server, credits[j], optype);
 			return PTR_ERR(midQ[i]);
 		}
 
@@ -883,17 +904,16 @@ compound_send_recv(const unsigned int xid, struct cifs_ses *ses,
 			if (midQ[i]->mid_state == MID_REQUEST_SUBMITTED) {
 				midQ[i]->mid_flags |= MID_WAIT_CANCELLED;
 				midQ[i]->callback = DeleteMidQEntry;
-				spin_unlock(&GlobalMid_Lock);
-				add_credits(ses->server, 1, optype);
-				return rc;
+				cancelled_mid[i] = true;
 			}
 			spin_unlock(&GlobalMid_Lock);
 		}
 	}
 
 	for (i = 0; i < num_rqst; i++)
-		if (midQ[i]->resp_buf)
-			credits += ses->server->ops->get_credits(midQ[i]);
+		if (!cancelled_mid[i] && midQ[i]->resp_buf
+		    && (midQ[i]->mid_state == MID_RESPONSE_RECEIVED))
+			credits[i] = ses->server->ops->get_credits(midQ[i]);
 
 	for (i = 0; i < num_rqst; i++) {
 		if (rc < 0)
@@ -901,8 +921,9 @@ compound_send_recv(const unsigned int xid, struct cifs_ses *ses,
 
 		rc = cifs_sync_mid_result(midQ[i], ses->server);
 		if (rc != 0) {
-			add_credits(ses->server, credits, optype);
-			return rc;
+			/* mark this mid as cancelled to not free it below */
+			cancelled_mid[i] = true;
+			goto out;
 		}
 
 		if (!midQ[i]->resp_buf ||
@@ -949,9 +970,11 @@ compound_send_recv(const unsigned int xid, struct cifs_ses *ses,
 	 * This is prevented above by using a noop callback that will not
 	 * wake this thread except for the very last PDU.
 	 */
-	for (i = 0; i < num_rqst; i++)
-		cifs_delete_mid(midQ[i]);
-	add_credits(ses->server, credits, optype);
+	for (i = 0; i < num_rqst; i++) {
+		if (!cancelled_mid[i])
+			cifs_delete_mid(midQ[i]);
+		add_credits(ses->server, credits[i], optype);
+	}
 
 	return rc;
 }