Commit 80ad0d4a authored Feb 10, 2015 by Sowmini Varadhan Committed by David S. Miller Feb 11, 2015

rds: rds_cong_queue_updates needs to defer the congestion update transmission



When the RDS transport is TCP, we cannot inline the call to rds_send_xmit
from rds_cong_queue_update because
(a) we are already holding the sock_lock in the recv path, and
    will deadlock when tcp_setsockopt/tcp_sendmsg try to get the sock
    lock
(b) cong_queue_update does an irqsave on the rds_cong_lock, and this
    will trigger warnings (for a good reason) from functions called
    out of sock_lock.

This patch reverts the change introduced by
2fa57129 ("RDS: Bypass workqueue when queueing cong updates").

The patch has been verified for both RDS/TCP as well as RDS/RDMA
to ensure that there are not regressions for either transport:
 - for verification of  RDS/TCP a client-server unit-test was used,
   with the server blocked in gdb and thus unable to drain its rcvbuf,
   eventually triggering a RDS congestion update.
 - for RDS/RDMA, the standard IB regression tests were used

Signed-off-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent bf250a1f

net/rds/cong.c

+15 −1

Original line number	Diff line number	Diff line
		@@ -221,7 +221,21 @@ void rds_cong_queue_updates(struct rds_cong_map *map)
		list_for_each_entry(conn, &map->m_conn_list, c_map_item) {
		if (!test_and_set_bit(0, &conn->c_map_queued)) {
		rds_stats_inc(s_cong_update_queued);
		rds_send_xmit(conn);
		/* We cannot inline the call to rds_send_xmit() here
		* for two reasons (both pertaining to a TCP transport):
		* 1. When we get here from the receive path, we
		* are already holding the sock_lock (held by
		* tcp_v4_rcv()). So inlining calls to
		* tcp_setsockopt and/or tcp_sendmsg will deadlock
		* when it tries to get the sock_lock())
		* 2. Interrupts are masked so that we can mark the
		* the port congested from both send and recv paths.
		* (See comment around declaration of rdc_cong_lock).
		* An attempt to get the sock_lock() here will
		* therefore trigger warnings.
		* Defer the xmit to rds_send_worker() instead.
		*/
		queue_delayed_work(rds_wq, &conn->c_send_w, 0);
		}
		}