FAILED: patch "[PATCH] tty: n_gsm: fix wrong DLCI release order" failed to apply to 4.19-stable tree

FAILED: patch "[PATCH] tty: n_gsm: fix wrong DLCI release order" failed to apply to 4.19-stable tree

[gregkh]

The patch below does not apply to the 4.19-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 deefc58bafb4841df7f0a0d85d89a1c819db9743 Mon Sep 17 00:00:00 2001
From: Daniel Starke <daniel.starke@siemens.com>
Date: Thu, 14 Apr 2022 02:42:14 -0700
Subject: [PATCH] tty: n_gsm: fix wrong DLCI release order

The current DLCI release order starts with the control channel followed by
the user channels. Reverse this order to keep the control channel open
until all user channels have been released.

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/20220414094225.4527-9-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 cc90b03ce005..6b953dfbb155 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -2146,8 +2146,8 @@ static void gsm_cleanup_mux(struct gsm_mux *gsm, bool disc)
 	/* Finish outstanding timers, making sure they are done */
 	del_timer_sync(&gsm->t2_timer);
 
-	/* Free up any link layer users */
-	for (i = 0; i < NUM_DLCI; i++)
+	/* Free up any link layer users and finally the control channel */
+	for (i = NUM_DLCI - 1; i >= 0; i--)
 		if (gsm->dlci[i])
 			gsm_dlci_release(gsm->dlci[i]);
 	mutex_unlock(&gsm->mutex);