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

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Date:   Tue, 5 May 2020 14:58:18 +0200
From:   Pavel Machek <pavel@...x.de>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Pavel Machek <pavel@...x.de>,
        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 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?

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

	       			      		     		Pavel
-- 
DENX Software Engineering GmbH,      Managing Director: Wolfgang Denk
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany

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