netdev - [PATCH] rds: rds_cong_queue_updates needs to defer the congestion update transmission

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Message-ID: <20150210142214.GO337@oracle.com>
Date:	Tue, 10 Feb 2015 09:22:14 -0500
From:	Sowmini Varadhan <sowmini.varadhan@...cle.com>
To:	chien.yen@...cle.com, davem@...emloft.net
Cc:	rds-devel@....oracle.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, sowmini.varadhan@...cle.com,
	chuck.lever@...cle.com
Subject: [PATCH] rds: rds_cong_queue_updates needs to defer the congestion
 update transmission


This patch fixes a sock_lock deadlock in the rds_cong_queue_update path.

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.

Signed-off-by: Sowmini Varadhan <sowmini.varadhan@...cle.com>
---
 net/rds/cong.c |   16 +++++++++++++++-
 1 files changed, 15 insertions(+), 1 deletions(-)

diff --git a/net/rds/cong.c b/net/rds/cong.c
index e5b65ac..765d18f 100644
--- a/net/rds/cong.c
+++ b/net/rds/cong.c
@@ -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:
+			 * 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 rds_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);
 		}
 	}
 
-- 
1.7.1

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