linux-kernel - Re: [PATCH 4.19 28/37] dmaengine: dmatest: Fix iteration non-stop logic

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Date:   Tue, 5 May 2020 16:19:11 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Pavel Machek <pavel@...x.de>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Stable <stable@...r.kernel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Vinod Koul <vkoul@...nel.org>
Subject: Re: [PATCH 4.19 28/37] dmaengine: dmatest: Fix iteration non-stop logic

On Tue, May 5, 2020 at 3:58 PM Pavel Machek <pavel@...x.de> wrote:
> On Tue 2020-05-05 15:51:16, Andy Shevchenko wrote:
> > On Tue, May 5, 2020 at 3:37 PM Pavel Machek <pavel@...x.de> wrote:
> > > > So, to the point, the conditional of checking the thread to be stopped being
> > > > first part of conjunction logic prevents to check iterations. Thus, we have to
> > > > always check both conditions to be able to stop after given
> > > > iterations.
> > >
> > > I ... don't understand. AFAICT the code is equivalent. Both && and ||
> > > operators permit "short" execution... but second part of expression
> > > has no sideeffects, so...
> >
> > ..
> >
> > > You are changing !a & !b into !(a | b). But that's equivalent
> > > expression. I hate to admit, but I had to draw truth table to prove
> > > that.
> ...
> > > What am I missing?
> >
> > Basic stuff. Compiler doesn't consider second part of conjunction when
> > first one (see operator precedence) is already false, so, it means:
> >
> > a & b
> > 0   x -> false
> > 1   0 -> false
> > 1   1 -> true
> >
> > x is not being considered at all. So, logically it's equivalent,
> > run-time it's not.
>
> Yeah, I pointed that out above. Both && and || permit short
> execution. But that does not matter, as neither "params->iterations"
> nor "total_tests >= params->iterations" have side effects.
>
> Where is the runtime difference?

We have to check *both* conditions. If we don't check iterations, we
just wait indefinitely until somebody tells us to stop.
Everything in the commit message and mentioned there commit IDs which
you may check.

> -       while (!kthread_should_stop()
> -              && !(params->iterations && total_tests >=
> -              params->iterations)) {
> +       while (!(kthread_should_stop() ||
> +              (params->iterations && total_tests >= params->iterations))) {

-- 
With Best Regards,
Andy Shevchenko