linux-kernel - [PATCH] unix: use wq_has_sleeper in unix_dgram

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Date:	Thu, 26 Nov 2015 19:23:15 +0000
From:	Rainer Weikusat <rweikusat@...ileactivedefense.com>
To:	davem@...emloft.net
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
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@...ileactivedefense.com>
---
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 4e95bdf..7aba73e 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -2057,8 +2057,10 @@ 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_wq))
+		wake_up_interruptible_sync_poll(&u->peer_wait,
+						POLLOUT | POLLWRNORM |
+						POLLWRBAND);
 
 	if (msg->msg_name)
 		unix_copy_addr(msg, skb->sk);
--
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