linux-kernel - Re: [PATCH] rds: rds_cong_queue_updates needs to defer the congestion update transmission

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Date:	Tue, 10 Feb 2015 11:50:39 -0500
From:	Chuck Lever <chuck.lever@...cle.com>
To:	Sowmini Varadhan <sowmini.varadhan@...cle.com>
Cc:	Chien Yen <chien.yen@...cle.com>, davem@...emloft.net,
	rds-devel@....oracle.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rds: rds_cong_queue_updates needs to defer the congestion update transmission


On Feb 10, 2015, at 9:22 AM, Sowmini Varadhan <sowmini.varadhan@...cle.com> wrote:

> 
> This patch fixes a sock_lock deadlock in the rds_cong_queue_update path.

Note that the deadlock appears to exist only with TCP transports.

> 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
> 

--
Chuck Lever
chuck[dot]lever[at]oracle[dot]com



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