netdev - Re: can: isotp: epoll breaks isotp

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Date: Tue, 18 Jul 2023 18:07:33 +0800
From: Hillf Danton <hdanton@...a.com>
To: Maxime Jayat <maxime.jayat@...ile-devices.fr>
Cc: Michal Sojka <michal.sojka@...t.cz>,
	Oliver Hartkopp <socketcan@...tkopp.net>,
	Marc Kleine-Budde <mkl@...gutronix.de>,
	linux-can@...r.kernel.org,
	netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	"Dae R. Jeong" <threeearcat@...il.com>,
	Hillf Danton <hdanton@...a.com>
Subject: Re: can: isotp: epoll breaks isotp_sendmsg

On Fri, Jun 30 2023, Maxime Jayat wrote:
> Hi,
>
> There is something not clear happening with the non-blocking behavior
> of ISO-TP sockets in the TX path, but more importantly, using epoll now
> completely breaks isotp_sendmsg.
> I believe it is related to
> 79e19fa79c ("can: isotp: isotp_ops: fix poll() to not report false 
> EPOLLOUT events"),
> but actually is probably deeper than that.
>
> I don't completely understand what is exactly going on, so I am sharing
> the problem I face:
>
> With an ISO-TP socket in non-blocking mode, using epoll seems to make
> isotp_sendmsg always return -EAGAIN.

Problem 1.

>
> By reverting 79e19fa79c, I get better results but still incorrect:

[...]

> It is then possible to write on the socket but the write is blocking,
> which is not the expected behavior for a non-blocking socket.

Problem 2.

My two cents with the two problems addressed.

--- x/net/can/isotp.c
+++ y/net/can/isotp.c
@@ -954,21 +954,18 @@ static int isotp_sendmsg(struct socket *
 	if (!so->bound || so->tx.state == ISOTP_SHUTDOWN)
 		return -EADDRNOTAVAIL;
 
-wait_free_buffer:
 	/* we do not support multiple buffers - for now */
-	if (wq_has_sleeper(&so->wait) && (msg->msg_flags & MSG_DONTWAIT))
-		return -EAGAIN;
-
-	/* wait for complete transmission of current pdu */
-	err = wait_event_interruptible(so->wait, so->tx.state == ISOTP_IDLE);
-	if (err)
-		goto err_event_drop;
-
-	if (cmpxchg(&so->tx.state, ISOTP_IDLE, ISOTP_SENDING) != ISOTP_IDLE) {
+	while (cmpxchg(&so->tx.state, ISOTP_IDLE, ISOTP_SENDING) != ISOTP_IDLE) {
 		if (so->tx.state == ISOTP_SHUTDOWN)
 			return -EADDRNOTAVAIL;
 
-		goto wait_free_buffer;
+		if (msg->msg_flags & MSG_DONTWAIT)
+			return -EAGAIN;
+
+		/* wait for complete transmission of current pdu */
+		err = wait_event_interruptible(so->wait, so->tx.state == ISOTP_IDLE);
+		if (err)
+			return err;
 	}
 
 	/* PDU size > default => try max_pdu_size */
--