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>

1 files changed, 4 insertions, 2 deletions

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index efb706e1d1c0..ac011b97097d 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);