cifs: prevent starvation in wait_for_free_credits for multi-credit requests

Reserve the last MAX_COMPOUND credits for any request asking for >1 credit.
This is to prevent future compound requests from becoming starved while waiting
for potentially many requests is there is a large number of concurrent
singe-credit requests.

However, we need to protect from servers that are very slow to hand out
new credits on new sessions so we only do this IFF there are 2*MAX_COMPOUND
(arbitrary) credits already in flight.

Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>

diff --git a/fs/cifs/transport.c b/fs/cifs/transport.c
index 1951f9f..9e08ce7 100644 (file)
--- a/fs/cifs/transport.c
+++ b/fs/cifs/transport.c
@@ -529,6 +529,34 @@ wait_for_free_credits(struct TCP_Server_Info *server, const int num_credits,
                        }
 
                        /*
+                        * For normal commands, reserve the last MAX_COMPOUND
+                        * credits to compound requests.
+                        * Otherwise these compounds could be permanently
+                        * starved for credits by single-credit requests.
+                        *
+                        * To prevent spinning CPU, block this thread until
+                        * there are >MAX_COMPOUND credits available.
+                        * But only do this is we already have a lot of
+                        * credits in flight to avoid triggering this check
+                        * for servers that are slow to hand out credits on
+                        * new sessions.
+                        */
+                       if (!optype && num_credits == 1 &&
+                           server->in_flight > 2 * MAX_COMPOUND &&
+                           *credits <= MAX_COMPOUND) {
+                               spin_unlock(&server->req_lock);
+                               cifs_num_waiters_inc(server);
+                               rc = wait_event_killable(server->request_q,
+                                       has_credits(server, credits,
+                                                   MAX_COMPOUND + 1));
+                               cifs_num_waiters_dec(server);
+                               if (rc)
+                                       return rc;
+                               spin_lock(&server->req_lock);
+                               continue;
+                       }
+
+                       /*
                         * Can not count locking commands against total
                         * as they are allowed to block on server.
                         */