FAILED: patch "[PATCH] tty: n_gsm: fix mux cleanup after unregister tty device" failed to apply to 4.9-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 4.9-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 284260f278b706364fb4c88a7b56ba5298d5973c Mon Sep 17 00:00:00 2001
From: Daniel Starke <daniel.starke@siemens.com>
Date: Thu, 14 Apr 2022 02:42:09 -0700
Subject: [PATCH] tty: n_gsm: fix mux cleanup after unregister tty device

Internally, we manage the alive state of the mux channels and mux itself
with the field member 'dead'. This makes it possible to notify the user
if the accessed underlying link is already gone. On the other hand,
however, removing the virtual ttys before terminating the channels may
result in peer messages being received without any internal target. Move
the mux cleanup procedure from gsmld_detach_gsm() to gsmld_close() to fix
this by keeping the virtual ttys open until the mux has been cleaned up.

Fixes: e1eaea46bb40 ("tty: n_gsm line discipline")
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20220414094225.4527-4-daniel.starke@siemens.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index f546dfe03d29..de97a3810731 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -2479,7 +2479,6 @@ static void gsmld_detach_gsm(struct tty_struct *tty, struct gsm_mux *gsm)
 		for (i = 1; i < NUM_DLCI; i++)
 			tty_unregister_device(gsm_tty_driver, base + i);
 	}
-	gsm_cleanup_mux(gsm, false);
 	tty_kref_put(gsm->tty);
 	gsm->tty = NULL;
 }
@@ -2544,6 +2543,12 @@ static void gsmld_close(struct tty_struct *tty)
 {
 	struct gsm_mux *gsm = tty->disc_data;
 
+	/* The ldisc locks and closes the port before calling our close. This
+	 * means we have no way to do a proper disconnect. We will not bother
+	 * to do one.
+	 */
+	gsm_cleanup_mux(gsm, false);
+
 	gsmld_detach_gsm(tty, gsm);
 
 	gsmld_flush_buffer(tty);