linux-kernel - Re: [PATCH v3 1/6] watchdog: Add API to trigger reboots

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Message-ID: <20140515214743.GA15577@roeck-us.net>
Date:	Thu, 15 May 2014 14:47:43 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
Cc:	linux-watchdog@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	Wim Van Sebroeck <wim@...ana.be>,
	Catalin Marinas <catalin.marinas@....com>,
	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	Will Deacon <will.deacon@....com>,
	Arnd Bergmann <arnd@...db.de>,
	Heiko Stuebner <heiko@...ech.de>,
	Russell King <linux@....linux.org.uk>,
	Jonas Jensen <jonas.jensen@...il.com>,
	Randy Dunlap <rdunlap@...radead.org>,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/6] watchdog: Add API to trigger reboots

On Thu, May 15, 2014 at 09:50:20PM +0100, One Thousand Gnomes wrote:
> > +void watchdog_do_reboot(void)
> > +{
> > +	if (wdd_reboot_dev)
> > +		wdd_reboot_dev->ops->reboot(wdd_reboot_dev);
> > +}
> > +EXPORT_SYMBOL(watchdog_do_reboot);
> 
> Crashes and burns if you are unloading a watchdog just as you try to
> reboot. Yes its wildly unlikely but it's still conceptually wrong.
> 
Possibly, but how is it different to the code it replaces ?

> >  
> > +	if (wdd->ops->reboot)
> > +		wdd_reboot_dev = wdd;
> > +
> 
> Two parallel registers from different bus types, parallel
> register/unregister ?
> 
Sorry, you lost me. What different bus types ?

> This feels to me like a backward step. We've gone from various device
> bits leaking into the core code (where they can work all the time) to
> various core code leaking into the devices which is asking for init order
> problems and other races.
> 
> Given we are talking about stuff of the order of 10-20 instructions I
> think duplication is not only the lesser evil it's also smaller, more
> reliable and easier to maintain.
> 
> IMHO this is a solution looking for a problem.
> 
Really ?  To me it seems to be much cleaner than setting the pointer to
arm_pm_restart directly from individual watchdog drivers. Also, and I was
told that other HW may benefit from it as well.

Do I understand it correctly that you prefer watchdog drivers to set
arm_pm_restart directly ? Maybe you can explain a bit why you believe
that to be a superior solution.

In addition to that, while I could obviously add some locking around access to
wdd_reboot_dev, existing code doesn't lock any changes to arm_pm_restart. I am
somewhat at loss why setting or clearing arm_pm_restart is less of a problem
that setting wdd_reboot_dev.

Thanks,
Guenter
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