[PATCH net-next 3/3] net: qrtr: implement shutdown()

[Chunkai Deng <chunkai.deng@oss.qualcomm.com>]

QRTR uses sock_no_shutdown(), so shutdown() fails with -EOPNOTSUPP and
there is no way to wake up a thread blocked in recv() on the socket.

Implement it the same way as unix_shutdown(): map SHUT_* to the
*_SHUTDOWN bits, set them in sk_shutdown and call sk_state_change().

Do not use lock_sock() here. qrtr_sendmsg() holds it while sleeping in
qrtr_tx_wait() until the remote returns flow control credit, which would
leave shutdown() stuck in an uninterruptible wait. Take sk_lock.slock to
serialize concurrent callers instead, and use WRITE_ONCE() for the
lockless readers. Do the same for the store in qrtr_release().

A sender already sleeping in qrtr_tx_wait() is not woken by SHUT_WR;
later sendmsg() calls fail with -EPIPE.

Signed-off-by: Chunkai Deng <chunkai.deng@oss.qualcomm.com>
---
 net/qrtr/af_qrtr.c | 35 +++++++++++++++++++++++++++++++++--
 1 file changed, 33 insertions(+), 2 deletions(-)

diff --git a/net/qrtr/af_qrtr.c b/net/qrtr/af_qrtr.c
index 09c1cb443f5c..c955c05e71bd 100644
--- a/net/qrtr/af_qrtr.c
+++ b/net/qrtr/af_qrtr.c
@@ -4,6 +4,7 @@
  * Copyright (c) 2013, The Linux Foundation. All rights reserved.
  */
 #include <linux/module.h>
+#include <linux/net.h>
 #include <linux/netlink.h>
 #include <linux/qrtr.h>
 #include <linux/termios.h>	/* For TIOCINQ/OUTQ */
@@ -1193,6 +1194,36 @@ static int qrtr_ioctl(struct socket *sock, unsigned int cmd, unsigned long arg)
 	return rc;
 }
 
+static int qrtr_shutdown(struct socket *sock, int mode)
+{
+	struct sock *sk;
+
+	if (mode < SHUT_RD || mode > SHUT_RDWR)
+		return -EINVAL;
+
+	/*
+	 * This maps:
+	 * SHUT_RD   (0) -> RCV_SHUTDOWN  (1)
+	 * SHUT_WR   (1) -> SEND_SHUTDOWN (2)
+	 * SHUT_RDWR (2) -> SHUTDOWN_MASK (3)
+	 */
+	mode++;
+
+	sk = sock->sk;
+
+	/*
+	 * Do not use lock_sock() here: sendmsg() holds it while sleeping on
+	 * remote flow control, which would block shutdown() indefinitely.
+	 * The spinlock only serializes concurrent shutdown() callers.
+	 */
+	spin_lock_bh(&sk->sk_lock.slock);
+	WRITE_ONCE(sk->sk_shutdown, sk->sk_shutdown | mode);
+	spin_unlock_bh(&sk->sk_lock.slock);
+	sk->sk_state_change(sk);
+
+	return 0;
+}
+
 static int qrtr_release(struct socket *sock)
 {
 	struct sock *sk = sock->sk;
@@ -1204,7 +1235,7 @@ static int qrtr_release(struct socket *sock)
 	lock_sock(sk);
 
 	ipc = qrtr_sk(sk);
-	sk->sk_shutdown = SHUTDOWN_MASK;
+	WRITE_ONCE(sk->sk_shutdown, SHUTDOWN_MASK);
 	if (!sock_flag(sk, SOCK_DEAD))
 		sk->sk_state_change(sk);
 
@@ -1237,7 +1268,7 @@ static const struct proto_ops qrtr_proto_ops = {
 	.ioctl		= qrtr_ioctl,
 	.gettstamp	= sock_gettstamp,
 	.poll		= datagram_poll,
-	.shutdown	= sock_no_shutdown,
+	.shutdown	= qrtr_shutdown,
 	.release	= qrtr_release,
 	.mmap		= sock_no_mmap,
 };

-- 
2.43.0