linux-kernel - Re: [v2] Enhanced support for MPC8xx/8xxx watchdog

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Message-ID: <20130625230431.GA29393@home.buserror.net>
Date:	Tue, 25 Jun 2013 18:04:31 -0500
From:	Scott Wood <scottwood@...escale.com>
To:	LEROY Christophe <christophe.leroy@....fr>
CC:	Wim Van Sebroeck <wim@...ana.be>, <linuxppc-dev@...ts.ozlabs.org>,
	<linux-kernel@...r.kernel.org>, <linux-watchdog@...r.kernel.org>
Subject: Re: [v2] Enhanced support for MPC8xx/8xxx watchdog

On Thu, Feb 28, 2013 at 09:52:22AM +0100, LEROY Christophe wrote:
> This patch modifies the behaviour of the MPC8xx/8xxx watchdog. On the MPC8xx,
> at 133Mhz, the maximum timeout of the watchdog timer is 1s, which means it must
> be pinged twice a second. This is not in line with the Linux watchdog concept
> which is based on a default watchdog timeout around 60s.
> This patch introduces an intermediate layer between the CPU and the userspace.
> The kernel pings the watchdog at the required frequency at the condition that
> userspace tools refresh it regularly.
> Existing parameter 'timeout' is renamed 'hw_time'.
> The new parameter 'timeout' allows to set up the userspace timeout.
> The driver also implements the WDIOC_SETTIMEOUT ioctl.
> 
> Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
> 
> 
> diff -ur linux-3.7.9/drivers/watchdog/mpc8xxx_wdt.c linux/drivers/watchdog/mpc8xxx_wdt.c
> --- linux-3.7.9/drivers/watchdog/mpc8xxx_wdt.c	2013-02-17 19:53:32.000000000 +0100
> +++ linux/drivers/watchdog/mpc8xxx_wdt.c	2013-02-27 16:00:07.000000000 +0100
> @@ -52,10 +52,17 @@
>  static struct mpc8xxx_wdt __iomem *wd_base;
>  static int mpc8xxx_wdt_init_late(void);
>  
> -static u16 timeout = 0xffff;
> -module_param(timeout, ushort, 0);
> +#define WD_TIMO 10			/* Default timeout = 10 seconds */

If the default Linux watchdog timeout is normally 60 seconds, why is it 10
here?

> +static uint timeout = WD_TIMO;
> +module_param(timeout, uint, 0);
>  MODULE_PARM_DESC(timeout,
> -	"Watchdog timeout in ticks. (0<timeout<65536, default=65535)");
> +	"Watchdog SW timeout in seconds. (0 < timeout < 65536s, default = "
> +				__MODULE_STRING(WD_TIMO) "s)");
> +static u16 hw_timo = 0xffff;
> +module_param(hw_timo, ushort, 0);
> +MODULE_PARM_DESC(hw_timo,
> +	"Watchdog HW timeout in ticks. (0 < hw_timo < 65536, default = 65535)");

hw_timeout would be more legibile -- this is a public interface.

>  static bool reset = 1;
>  module_param(reset, bool, 0);
> @@ -72,10 +79,12 @@
>   * to 0
>   */
>  static int prescale = 1;
> -static unsigned int timeout_sec;
> +static unsigned int hw_timo_sec;
>  
> +static int wdt_auto = 1;

bool, and add a comment indicating what this means.

>  static unsigned long wdt_is_open;
>  static DEFINE_SPINLOCK(wdt_spinlock);
> +static unsigned long wdt_last_ping;
>  
>  static void mpc8xxx_wdt_keepalive(void)
>  {
> @@ -91,9 +100,20 @@
>  
>  static void mpc8xxx_wdt_timer_ping(unsigned long arg)
>  {
> -	mpc8xxx_wdt_keepalive();
> -	/* We're pinging it twice faster than needed, just to be sure. */
> -	mod_timer(&wdt_timer, jiffies + HZ * timeout_sec / 2);
> +	if (wdt_auto)
> +		wdt_last_ping = jiffies;
> +
> +	if (jiffies - wdt_last_ping <= timeout * HZ) {

So timeout cannot be more than UINT_MAX / HZ...  Might want to check for
that, just in case.

What happens if there's a race?  If another CPU updates wdt_last_ping in
parallel, then you could see wdt_last_ping greater than the value you
read for jiffies.  Since this is an unsigned comparison, it will fail to
call keepalive.  You might get saved by pinging it twice as often as
necessary, but you shouldn't rely on that.

> +		mpc8xxx_wdt_keepalive();
> +		/* We're pinging it twice faster than needed, to be sure. */
> +		mod_timer(&wdt_timer, jiffies + HZ * hw_timo_sec / 2);
> +	}
> +}
> +
> +static void mpc8xxx_wdt_sw_keepalive(void)
> +{
> +	wdt_last_ping = jiffies;
> +	mpc8xxx_wdt_timer_ping(0);
>  }

This isn't new with this patch, but it looks like
mpc8xxx_wdt_keepalive() can be called either from timer context, or with
interrupts enabled... yet it uses a bare spin_lock() rather than an
irq-safe version.  This should be fixed.

-Scott

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