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From:   Ben Hutchings <ben@decadent.org.uk>
To:     linux-kernel@vger.kernel.org, stable@vger.kernel.org
CC:     akpm@linux-foundation.org,
        "Geert Uytterhoeven" <geert+renesas@glider.be>,
        "Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
        "Ulrich Hecht" <ulrich.hecht+renesas@gmail.com>,
        "Nguyen Viet Dung" <dung.nguyen.aj@renesas.com>,
        "Yoshihiro Shimoda" <yoshihiro.shimoda.uh@renesas.com>
Date:   Thu, 07 Jun 2018 15:05:21 +0100
Message-ID: <lsq.1528380321.197204780@decadent.org.uk>
Subject: [PATCH 3.16 308/410] serial: sh-sci: prevent lockup on full TTY
 buffers
In-Reply-To: <lsq.1528380320.647747352@decadent.org.uk>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

3.16.57-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>

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.

Reported-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Nguyen Viet Dung <dung.nguyen.aj@renesas.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
 drivers/tty/serial/sh-sci.c | 2 ++
 1 file changed, 2 insertions(+)

--- a/drivers/tty/serial/sh-sci.c
+++ b/drivers/tty/serial/sh-sci.c
@@ -734,6 +734,8 @@ 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 */
 		serial_port_out(port, SCxSR, SCxSR_RDxF_CLEAR(port));
 	}