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

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Date:   Wed, 6 May 2020 00:37:11 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Pavel Machek <pavel@...x.de>
Cc:     Sasha Levin <sashal@...nel.org>,
        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 6:57 PM Pavel Machek <pavel@...x.de> wrote:
> On Tue 2020-05-05 11:32:27, Sasha Levin wrote:
> > On Tue, May 05, 2020 at 05:05:37PM +0300, Andy Shevchenko wrote:

...

> > I'm a bit confused about this too. Maybe it's too early in the morning,
> > so I wrote this little test program:
> >
> > #include <stdio.h>
> > #include <stdlib.h>
> >
> > int main(int argc, char *argv[])
> > {
> >        int a = atoi(argv[1]);
> >        int b = atoi(argv[2]);
> >
> >        if (!a && !b)
> >                printf("A");
> >        else
> >                printf("B");
> >
> >        if (!(a || b))
> >                printf("A");
> >        else
> >                printf("B");
> >
> >        printf("\n");
> >
> >        return 0;
> > }
> >
> > Andy, could you give an example of two values which will print something
> > other than "AA" or "BB"?
>
> The issue here is "sideffects". Does b have to be evaluated at all?
> There is no difference between
>
>       int a, b;
>       if (a && b)
>
> and
>
>         if ((!!a) & (!!b))
> .
>
> But there would be difference between
>
>       int a, b;
>         if (a && b++)
>
> and
>         if ((!!a) & (!!(b++)))
>
> But:
>
> 1) && and || behave same way w.r.t. side effects
>
> 2) in the patch we are talking about b has no important side effects

I have to admit that this seems like a luck and the real issue somewhere else.
Definitely another test should be performed.

Thank you, Pavel, for pointing this out.

-- 
With Best Regards,
Andy Shevchenko