linux-kernel - Re: [RFC: PATCH 2/2] iio: adc: exynos_adc: Handle timeout and race conditions

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Message-ID: <CAD=FV=V+-DwCjGoookySCazazBBYOoRO+=G5Q_WXAw+SrXNc-A@mail.gmail.com>
Date:	Fri, 15 Mar 2013 17:37:54 -0700
From:	Doug Anderson <dianders@...omium.org>
To:	Lars-Peter Clausen <lars@...afoo.de>
Cc:	Naveen Krishna Chatradhi <ch.naveen@...sung.com>,
	linux-iio <linux-iio@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-samsung-soc@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Naveen Krishna <naveenkrishna.ch@...il.com>
Subject: Re: [RFC: PATCH 2/2] iio: adc: exynos_adc: Handle timeout and race conditions

On Fri, Mar 15, 2013 at 2:53 PM, Lars-Peter Clausen <lars@...afoo.de> wrote:
> What exactly is the spinlock protecting against here? Concurrent runs of
> exynos_adc_isr? This is probably not issue in the first place.
>
> What you want to protect against is that completion is completed between the
> call to INIT_COMPLETION() and the start of a new conversion. So the sections
> that need to be under the spinlock are the complete call here and the point
> from INIT_COMPLETION until the transfer is started in exynos_read_raw(). Make
> sure to use spin_lock_irq there.

...and at that point I _think_ you won't also need the mutex.

A reasonable way to test to see if you've got this all correct would be to:

* Start two processes that are reading from different ADCs that will
report very different values (maybe add a device tree node for adc1 or
adc7 and use those since they're not really connected to
thermistors?).

* Have your two processes read as fast as they can.  This could just
be "while true; do cat /sys/class/hwmon/hwmon0/device/temp1_input;
done"

* Decrease your timeout and maybe(?) sprinkle some random udelays in
the irq handler so that the timeouts happen sometimes but not others.

* Periodically cancel one of the readers with Ctrl-C

If all is working well then you should always get back the right value
from the right reader (and get no crashes).

-Doug
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