Patch "TTY: n_gsm, fix false positive WARN_ON" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

TTY: n_gsm, fix false positive WARN_ON

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
tty-n_gsm-fix-false-positive-warn_on.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Jiri Slaby <[email protected]>
Date: Tue, 22 Mar 2016 18:09:51 +0100
Subject: TTY: n_gsm, fix false positive WARN_ON

From: Jiri Slaby <[email protected]>

commit d175feca89a1c162f60f4e3560ca7bc9437c65eb upstream.

Dmitry reported, that the current cleanup code in n_gsm can trigger a warning:

WARNING: CPU: 2 PID: 24238 at drivers/tty/n_gsm.c:2048 gsm_cleanup_mux+0x166/0x6b0()
...
Call Trace:
...
 [<ffffffff81247ab9>] warn_slowpath_null+0x29/0x30 kernel/panic.c:490
 [<ffffffff828d0456>] gsm_cleanup_mux+0x166/0x6b0 drivers/tty/n_gsm.c:2048
 [<ffffffff828d4d87>] gsmld_open+0x5b7/0x7a0 drivers/tty/n_gsm.c:2386
 [<ffffffff828b9078>] tty_ldisc_open.isra.2+0x78/0xd0 drivers/tty/tty_ldisc.c:447
 [<ffffffff828b973a>] tty_set_ldisc+0x1ca/0xa70 drivers/tty/tty_ldisc.c:567
 [<     inline     >] tiocsetd drivers/tty/tty_io.c:2650
 [<ffffffff828a14ea>] tty_ioctl+0xb2a/0x2140 drivers/tty/tty_io.c:2883

...

But this is a legal path when open fails to find a space in the gsm_mux array and tries to clean up. So make it a standard test instead of a warning.

---
 drivers/tty/n_gsm.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

@@ -2045,7 +2045,9 @@ static void gsm_cleanup_mux(struct gsm_m
 		}
 	}
 	spin_unlock(&gsm_mux_lock);
-	WARN_ON(i == MAX_MUX);
+	/* open failed before registering => nothing to do */
+	if (i == MAX_MUX)
+		return;
 
 	/* In theory disconnecting DLCI 0 is sufficient but for some
 	   modems this is apparently not the case. */

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/bluetooth-vhci-purge-unhandled-skbs.patch queue-4.4/tty-n_gsm-fix-false-positive-warn_on.patch queue-4.4/bluetooth-vhci-fix-open_timeout-vs.-hdev-race.patch queue-4.4/tty-vt-return-error-when-con_startup-fails.patch