can: can327: flush TX_work on ldisc .close()

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2071839
Tested: This is one of a series of patch sets to enable Arm SystemReady IR
 support.  This set updates the CAN driver subsystem.  This set has
 been tested on NXP imx8mm and NV Jetson Xavier NX via simple boot
 tests, using cansend/candump from can-utils to pass messages through
 a virtual CAN interface, and of course the CI loop.

commit f4a4d12
Author: Max Staudt <max@enpas.org>
Date:   Sat Dec 3 01:01:48 2022 +0900

    can: can327: flush TX_work on ldisc .close()

    Additionally, remove it from .ndo_stop().

    This ensures that the worker is not called after being freed, and that
    the UART TX queue remains active to send final commands when the
    netdev is stopped.

    Thanks to Jiri Slaby for finding this in slcan:

      https://lore.kernel.org/linux-can/20221201073426.17328-1-jirislaby@kernel.org/

    A variant of this patch for slcan, with the flush in .ndo_stop() still
    present, has been tested successfully on physical hardware:

      https://bugzilla.suse.com/show_bug.cgi?id=1205597

    Fixes: 43da2f0 ("can: can327: CAN/ldisc driver for ELM327 based OBD-II adapters")
    Cc: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
    Cc: Max Staudt <max@enpas.org>
    Cc: Wolfgang Grandegger <wg@grandegger.com>
    Cc: Marc Kleine-Budde <mkl@pengutronix.de>
    Cc: "David S. Miller" <davem@davemloft.net>
    Cc: Eric Dumazet <edumazet@google.com>
    Cc: Jakub Kicinski <kuba@kernel.org>
    Cc: Paolo Abeni <pabeni@redhat.com>
    Cc: linux-can@vger.kernel.org
    Cc: netdev@vger.kernel.org
    Cc: stable@vger.kernel.org
    Signed-off-by: Max Staudt <max@enpas.org>
    Link: https://lore.kernel.org/all/20221202160148.282564-1-max@enpas.org
    Cc: stable@vger.kernel.org
    Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
    (cherry picked from commit f4a4d12)

Signed-off-by: Al Stone <ahs3@redhat.com>

Author: Al Stone

drivers/net/can/can327.c

@@ -796,9 +796,9 @@ static int can327_netdev_close(struct net_device *dev)
 
 	netif_stop_queue(dev);
 
-	/* Give UART one final chance to flush. */
-	clear_bit(TTY_DO_WRITE_WAKEUP, &elm->tty->flags);
-	flush_work(&elm->tx_work);
+	/* We don't flush the UART TX queue here, as we want final stop
+	 * commands (like the above dummy char) to be flushed out.
+	 */
 
 	can_rx_offload_disable(&elm->offload);
 	elm->can.state = CAN_STATE_STOPPED;
@@ -1069,12 +1069,15 @@ static void can327_ldisc_close(struct tty_struct *tty)
 {
 	struct can327 *elm = (struct can327 *)tty->disc_data;
 
-	/* unregister_netdev() calls .ndo_stop() so we don't have to.
-	 * Our .ndo_stop() also flushes the TTY write wakeup handler,
-	 * so we can safely set elm->tty = NULL after this.
-	 */
+	/* unregister_netdev() calls .ndo_stop() so we don't have to. */
 	unregister_candev(elm->dev);
 
+	/* Give UART one final chance to flush.
+	 * No need to clear TTY_DO_WRITE_WAKEUP since .write_wakeup() is
+	 * serialised against .close() and will not be called once we return.
+	 */
+	flush_work(&elm->tx_work);
+
 	/* Mark channel as dead */
 	spin_lock_bh(&elm->lock);
 	tty->disc_data = NULL;