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

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Date:   Tue, 5 May 2020 17:05:37 +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 4:37 PM Pavel Machek <pavel@...x.de> wrote:
> On Tue 2020-05-05 16:19:11, Andy Shevchenko wrote:
> > 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

vvv
>>>>>> to be able to stop after given iterations.
^^^

...

> > > 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.
>
> No.

Yes. Please, read carefully the commit message (for your convenience I
emphasized above). I don't want to spend time on this basics stuff
anymore.

> If kthread_should_stop() is true, we break the loop. Both old code and
> new code does that. Neither old nor new code checks the
> "params->iterations && total_tests >=dparams->iterations" condition,
> as both && and || do short execution).
>
> If you wanted both conditions to always evaluate, you'd have to do
>
> #       while (!kthread_should_stop()
> #              & !(params->iterations && total_tests >=
> #              params->iterations)) {
>
> (note && -> &). But, again, there's no reason to do that, as second
> part of expression does not have side effects.

It fixes a bug in the code, try with and without this change. (I can
reproduce it here)

-- 
With Best Regards,
Andy Shevchenko