Re: [PATCH v6 1/2] Use "request_muxed_region" in it87 watchdogdrivers

From: Guenter Roeck
Date: Thu Apr 14 2011 - 22:41:19 EST

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@xxxxxxxxxxxx> 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.

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