linux-kernel - Re: [PATCH v2] hwmon: (applesmc) fix UB and udelay overflow

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Date:   Wed, 2 Oct 2019 14:43:51 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     clang-built-linux <clang-built-linux@...glegroups.com>,
        jdelvare@...e.com,
        Tomasz Paweł Gajc <tpgxyz@...il.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Henrik Rydberg <rydberg@...math.org>,
        linux-hwmon@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] hwmon: (applesmc) fix UB and udelay overflow

On Mon, Sep 30, 2019 at 5:01 PM Guenter Roeck <linux@...ck-us.net> wrote:
>
> Again, I fail to understand why waiting for a multiple of 20 seconds
> under any circumstances would make any sense. Maybe the idea was
> to divide us by 1000 before entering the second loop ?

Yes, that's very clearly a mistake of mine.

>
> Looking into the code, there is no need to use udelay() in the first
> place. It should be possible to replace the longer waits with
> usleep_range(). Something like
>
>                 if (us < some_low_value)        // eg. 0x80
>                         delay(us)

Did you mean udelay here?

>                 else
>                         usleep_range(us, us * 2);
>
> should do, and at the same time prevent the system from turning
> into a space heater.

The issue would persist with the above if udelay remains in a loop
that gets fully unrolled.  That's while I "peel" the loop into two
loops over different ranges with different bodies.

I think I should iterate in the first loop until the number of `us` is
greater than 1000 (us per ms)(which is less of a magical constant and
doesn't expose internal implementation details of udelay), then start
the second loop (dividing us by 1000).  What do you think, Guenter?

--
Thanks,
~Nick Desaulniers