Subject: [PATCH v4] ipx: fix locking regression in ipx_sendmsg and ipx_recvmsg

From: Jiri Bohac <[email-masked]>
Date: Wed, 19 Nov 2014 23:05:49 +0100

This fixes an old regression introduced by commit
b0d0d915 (ipx: remove the BKL).

When a recvmsg syscall blocks waiting for new data, no data can be sent on the
same socket with sendmsg because ipx_recvmsg() sleeps with the socket locked.

This breaks mars-nwe (NetWare emulator):
- the ncpserv process reads the request using recvmsg
- ncpserv forks and spawns nwconn
- ncpserv calls a (blocking) recvmsg and waits for new requests
- nwconn deadlocks in sendmsg on the same socket 

Commit b0d0d915 has simply replaced BKL locking with
lock_sock/release_sock. Unlike now, BKL got unlocked while
sleeping, so a blocking recvmsg did not block a concurrent
sendmsg.

Only keep the socket locked while actually working with the socket data and
release it prior to calling skb_recv_datagram(). 

Signed-off-by: Jiri Bohac <[email-masked]>

diff --git a/net/ipx/af_ipx.c b/net/ipx/af_ipx.c
index a0c7536..d0725d9 100644
--- a/net/ipx/af_ipx.c
+++ b/net/ipx/af_ipx.c
@@ -1764,6 +1764,7 @@ static int ipx_recvmsg(struct kiocb *iocb, struct socket *sock,
 	struct ipxhdr *ipx = NULL;
 	struct sk_buff *skb;
 	int copied, rc;
+	bool locked = true;
 
 	lock_sock(sk);
 	/* put the autobinding in */
@@ -1790,6 +1791,8 @@ static int ipx_recvmsg(struct kiocb *iocb, struct socket *sock,
 	if (sock_flag(sk, SOCK_ZAPPED))
 		goto out;
 
+	release_sock(sk);
+	locked = false;
 	skb = skb_recv_datagram(sk, flags & ~MSG_DONTWAIT,
 				flags & MSG_DONTWAIT, &rc);
 	if (!skb) {
@@ -1825,7 +1828,8 @@ static int ipx_recvmsg(struct kiocb *iocb, struct socket *sock,
 out_free:
 	skb_free_datagram(sk, skb);
 out:
-	release_sock(sk);
+	if (locked)
+		release_sock(sk);
 	return rc;
 }
 
-- 
Jiri Bohac <[email-masked]>
SUSE Labs, SUSE CZ