From 77b75f4d8cf105b599beef38724f8171e557919d Mon Sep 17 00:00:00 2001
From: Rainer Weikusat <rweikusat@mobileactivedefense.com>
Date: Thu, 26 Nov 2015 19:23:15 +0000
Subject: [PATCH] unix: use wq_has_sleeper in unix_dgram_recvmsg

The current unix_dgram_recvmsg does a wake up for every received
datagram. This seems wasteful as only SOCK_DGRAM client sockets in an
n:1 association with a server socket will ever wait because of the
associated condition. The patch below changes the function such that the
wake up only happens if wq_has_sleeper indicates that someone actually
wants to be notified. Testing with SOCK_SEQPACKET and SOCK_DGRAM socket
seems to confirm that this is an improvment.

Signed-Off-By: Rainer Weikusat <rweikusat@mobileactivedefense.com>

Signed-off-by: David S. Miller <davem@davemloft.net>
---
 net/unix/af_unix.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index efb706e1d1c0c..ac011b97097d6 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -1914,8 +1914,10 @@ static int unix_dgram_recvmsg(struct socket *sock, struct msghdr *msg,
 		goto out_unlock;
 	}
 
-	wake_up_interruptible_sync_poll(&u->peer_wait,
-					POLLOUT | POLLWRNORM | POLLWRBAND);
+	if (wq_has_sleeper(&u->peer_wait))
+		wake_up_interruptible_sync_poll(&u->peer_wait,
+						POLLOUT | POLLWRNORM |
+						POLLWRBAND);
 
 	if (msg->msg_name)
 		unix_copy_addr(msg, skb->sk);
-- 
GitLab