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Commit 16b34aa4 authored by Ronnie Sahlberg's avatar Ronnie Sahlberg Committed by Steve French
Browse files

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: default avatarRonnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: default avatarSteve French <stfrench@microsoft.com>
Reviewed-by: default avatarPavel Shilovsky <pshilov@microsoft.com>
parent b227d215

fs/cifs/transport.c

+28 −0
Original line number Diff line number Diff line
@@ -528,6 +528,34 @@ wait_for_free_credits(struct TCP_Server_Info *server, const int num_credits, 
				return -ENOENT;

			}



			/*

			 * 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.