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

[Pavel Machek]

On Tue 2020-05-05 15:51:16, Andy Shevchenko wrote:
> On Tue, May 5, 2020 at 3:37 PM Pavel Machek <pavel@xxxxxxx> 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
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