cifs: prevent starvation in wait_for_free_credits for multi-credit requests (16b34aa4) · Commits · 戴 / test

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>

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.

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