linux-kernel - Re: [PATCH v6 1/2] Use "request_muxed

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Date:	Thu, 14 Apr 2011 19:40:05 -0700
From:	Guenter Roeck <guenter.roeck@...csson.com>
To:	Natarajan Gurumoorthy <natg@...gle.com>
CC:	Jean Delvare <khali@...ux-fr.org>,
	Wim Van Sebroeck <wim@...ana.be>,
	Mike Waychison <mikew@...gle.com>,
	"lm-sensors@...sensors.org" <lm-sensors@...sensors.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-watchdog@...r.kernel.org" <linux-watchdog@...r.kernel.org>
Subject: Re: [PATCH v6 1/2] Use "request_muxed_region" in it87 watchdog
 drivers

On Thu, Apr 14, 2011 at 09:18:57PM -0400, Natarajan Gurumoorthy wrote:
> On Thu, Apr 14, 2011 at 3:49 PM, Guenter Roeck
> <guenter.roeck@...csson.com> wrote:
> > This is way too complicated. Just return an error if
> > request_muxed_region fails, like all other callers of
> > request_muxed_region do.
> >
> Guenter,
>       This maybe a little complicated but it is a solution that will
> correctly deal with the following scenario:
>       Hardware: it8712f
>       cpu: Multicore
>       kernel: smp
>       Modules being loaded during initialization: it8712f_wtd and w83697hf
> 
> The problem is one of the cores is halfway through initializing
> it8712f_wdt and another core is just starting to run the
> "w83697hf_check_wdt" which is going to call "request_region" to
> reserve 0x2e 0x2f region. Just after that call on the other core
> it8712f_wdt_init calls it8712f_wdt_disable which calls superio_enter
> which will call "request_muxed_region" for the exact same region. This
> is  guaranteed to fail because as of now w83697hf is one of the many
> non compliant drivers who are calling "request_region". The w83697hf
> will do the probe and conclude there is no w83697hf chip in the system
> and abort rest of the driver initialization. Meanwhile the it8712f_wtd
> driver initialization will fail. This will be one of those
> intermittent failures that make most kernel device driver writers
> prematurely bald :-).
> 
> The way I have reworked the driver will survive the above scenario.
> 
Unless I am missing something, the problem is that there is another driver 
calling request_region() instead of request_muxed_region().

w83697hf_check_wdt() (and other drivers doing the same) should call
request_muxed_region(). Otherwise, the same problem could happen with the
caller(s) of request_region() - those calls could fail as well.
Plus, other callers of request_muxed_reason() not implementing
your retry code could end up with the same problem, ie with spurious failures.

I think we will have to find a solution which does not require retries
when calling request_muxed_region(). On the other side, others know
this code much better than I do, so maybe someone has a better solution.

Thanks,
Guenter
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