Patch "serial: sh-sci: prevent lockup on full TTY buffers" has been added to the 4.9-stable tree

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

serial: sh-sci: prevent lockup on full TTY buffers

to the 4.9-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:
serial-sh-sci-prevent-lockup-on-full-tty-buffers.patch
and it can be found in the queue-4.9 subdirectory.

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

From: Ulrich Hecht <[email protected]>
Date: Thu, 15 Feb 2018 13:02:27 +0100
Subject: serial: sh-sci: prevent lockup on full TTY buffers

From: Ulrich Hecht <[email protected]>

commit 7842055bfce4bf0170d0f61df8b2add8399697be upstream.

When the TTY buffers fill up to the configured maximum, a system lockup occurs:

[  598.820128] INFO: rcu_preempt detected stalls on CPUs/tasks:
[  598.825796]  0-...!: (1 GPs behind) idle=5a6/2/0 softirq=1974/1974 fqs=1
[  598.832577]  (detected by 3, t=62517 jiffies, g=296, c=295, q=126)
[  598.838755] Task dump for CPU 0:
[  598.841977] swapper/0       R  running task        0     0      0 0x00000022
[  598.849023] Call trace:
[  598.851476]  __switch_to+0x98/0xb0
[  598.854870]            (null)

This can be prevented by doing a dummy read of the RX data register.

This issue affects both HSCIF and SCIF ports. Reported for R-Car H3 ES2.0; reproduced and fixed on H3 ES1.1. Probably affects other R-Car platforms as well.

---
 drivers/tty/serial/sh-sci.c |    2 ++
 1 file changed, 2 insertions(+)

@@ -935,6 +935,8 @@ static void sci_receive_chars(struct uar
 		/* Tell the rest of the system the news. New characters! */
 		tty_flip_buffer_push(tport);
 	} else {
+		/* TTY buffers full; read from RX reg to prevent lockup */
+		serial_port_in(port, SCxRDR);
 		serial_port_in(port, SCxSR); /* dummy read */
 		sci_clear_SCxSR(port, SCxSR_RDxF_CLEAR(port));
 	}

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

queue-4.9/serial-sh-sci-prevent-lockup-on-full-tty-buffers.patch