Patch "serial: sh-sci: Remove cpufreq notifier to fix crash/deadlock" 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

serial: sh-sci: Remove cpufreq notifier to fix crash/deadlock

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:
serial-sh-sci-remove-cpufreq-notifier-to-fix-crash-deadlock.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: Geert Uytterhoeven <[email protected]>
Date: Tue, 5 Jan 2016 19:36:37 +0100
Subject: serial: sh-sci: Remove cpufreq notifier to fix crash/deadlock

From: Geert Uytterhoeven <[email protected]>

commit ff1cab374ad98f4b9f408525ca9c08992b4ed784 upstream.

The BSP team noticed that there is spin/mutex lock issue on sh-sci when CPUFREQ is used. The issue is that the notifier function may call mutex_lock() while the spinlock is held, which can lead to a BUG(). This may happen if CPUFREQ is changed while another CPU calls clk_get_rate().

Taking the spinlock was added to the notifier function in commit e552de2413edad1a ("sh-sci: add platform device private data"), to protect the list of serial ports against modification during traversal. At that time the Common Clock Framework didn't exist yet, and clk_get_rate() just returned clk->rate without taking a mutex. Note that since commit d535a2305facf9b4 ("serial: sh-sci: Require a device per port mapping."), there's no longer a list of serial ports to traverse, and taking the spinlock became superfluous.

To fix the issue, just remove the cpufreq notifier:
1. The notifier doesn't work correctly: all it does is update stored clock rates; it does not update the divider in the hardware. The divider will only be updated when calling sci_set_termios(). I believe this was broken back in 2004, when the old drivers/char/sh-sci.c driver (where the notifier did update the divider) was replaced by drivers/serial/sh-sci.c (where the notifier just updated port->uartclk).
Cfr. full-history-linux commits 6f8deaef2e9675d9 ("[PATCH] sh: port sh-sci driver to the new API") and 3f73fe878dc9210a ("[PATCH] Remove old sh-sci driver").
2. On modern SoCs, the sh-sci parent clock rate is no longer related to the CPU clock rate anyway, so using a cpufreq notifier is futile.

---
 drivers/tty/serial/sh-sci.c |   39 ---------------------------------------
 1 file changed, 39 deletions(-)

@@ -38,7 +38,6 @@
 #include <linux/major.h>
 #include <linux/module.h>
 #include <linux/mm.h>
-#include <linux/notifier.h>
 #include <linux/of.h>
 #include <linux/platform_device.h>
 #include <linux/pm_runtime.h>

@@ -116,8 +115,6 @@ struct sci_port {
 	struct timer_list		rx_timer;
 	unsigned int			rx_timeout;
 #endif
-
-	struct notifier_block		freq_transition;
 };
 
 #define SCI_NPORTS CONFIG_SERIAL_SH_SCI_NR_UARTS

@@ -1606,29 +1603,6 @@ static irqreturn_t sci_mpxed_interrupt(i
 	return ret;
 }
 
-/*
- * Here we define a transition notifier so that we can update all of our
- * ports' baud rate when the peripheral clock changes.
- */
-static int sci_notifier(struct notifier_block *self,
-			unsigned long phase, void *p)
-{
-	struct sci_port *sci_port;
-	unsigned long flags;
-
-	sci_port = container_of(self, struct sci_port, freq_transition);
-
-	if (phase == CPUFREQ_POSTCHANGE) {
-		struct uart_port *port = &sci_port->port;
-
-		spin_lock_irqsave(&port->lock, flags);
-		port->uartclk = clk_get_rate(sci_port->iclk);
-		spin_unlock_irqrestore(&port->lock, flags);
-	}
-
-	return NOTIFY_OK;
-}
-
 static const struct sci_irq_desc {
 	const char	*desc;
 	irq_handler_t	handler;

@@ -2559,9 +2533,6 @@ static int sci_remove(struct platform_de
 {
 	struct sci_port *port = platform_get_drvdata(dev);
 
-	cpufreq_unregister_notifier(&port->freq_transition,
-				    CPUFREQ_TRANSITION_NOTIFIER);
-
 	uart_remove_one_port(&sci_uart_driver, &port->port);
 
 	sci_cleanup_single(port);

@@ -2714,16 +2685,6 @@ static int sci_probe(struct platform_dev
 	if (ret)
 		return ret;
 
-	sp->freq_transition.notifier_call = sci_notifier;
-
-	ret = cpufreq_register_notifier(&sp->freq_transition,
-					CPUFREQ_TRANSITION_NOTIFIER);
-	if (unlikely(ret < 0)) {
-		uart_remove_one_port(&sci_uart_driver, &sp->port);
-		sci_cleanup_single(sp);
-		return ret;
-	}
-
 #ifdef CONFIG_SH_STANDARD_BIOS
 	sh_bios_gdb_detach();
 #endif

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

queue-4.4/serial-sh-sci-remove-cpufreq-notifier-to-fix-crash-deadlock.patch