linux-kernel - Re: [PATCH v5 6/9] bitops: let optimize out non-atomic bitops on compile-time constants

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Date:   Fri, 15 Jul 2022 07:19:12 -0700
From:   Yury Norov <yury.norov@...il.com>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Alexander Lobakin <alexandr.lobakin@...el.com>,
        Arnd Bergmann <arnd@...db.de>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Mark Rutland <mark.rutland@....com>,
        Matt Turner <mattst88@...il.com>,
        Brian Cain <bcain@...cinc.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>,
        "David S. Miller" <davem@...emloft.net>,
        Kees Cook <keescook@...omium.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Marco Elver <elver@...gle.com>, Borislav Petkov <bp@...e.de>,
        Tony Luck <tony.luck@...el.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, kernel test robot <lkp@...el.com>,
        linux-alpha@...r.kernel.org, linux-hexagon@...r.kernel.org,
        linux-ia64@...r.kernel.org, linux-m68k@...ts.linux-m68k.org,
        linux-sh@...r.kernel.org, sparclinux@...r.kernel.org,
        linux-arch@...r.kernel.org, llvm@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 6/9] bitops: let optimize out non-atomic bitops on
 compile-time constants

On Fri, Jul 15, 2022 at 06:49:46AM -0700, Guenter Roeck wrote:
> On 7/15/22 06:26, Alexander Lobakin wrote:
> > From: Guenter Roeck <linux@...ck-us.net>
> > Date: Thu, 14 Jul 2022 17:04:02 -0700
> > 
> > > On Fri, Jun 24, 2022 at 02:13:10PM +0200, Alexander Lobakin wrote:
> > > > Currently, many architecture-specific non-atomic bitop
> > > > implementations use inline asm or other hacks which are faster or
> > 
> > [...]
> > 
> > > > Cc: Mark Rutland <mark.rutland@....com>
> > > > Signed-off-by: Alexander Lobakin <alexandr.lobakin@...el.com>
> > > > Reviewed-by: Marco Elver <elver@...gle.com>
> > > 
> > > Building i386:allyesconfig ... failed
> > > --------------
> > > Error log:
> > > arch/x86/platform/olpc/olpc-xo1-sci.c: In function 'send_ebook_state':
> > > arch/x86/platform/olpc/olpc-xo1-sci.c:83:63: error: logical not is only applied to the left hand side of comparison
> > 
> > Looks like a trigger, not a cause... Anyway, this construct:
> > 
> > 	unsigned char state;
> > 
> > 	[...]
> > 
> > 	if (!!test_bit(SW_TABLET_MODE, ebook_switch_idev->sw) == state)
> > 
> > doesn't look legit enough.
> > That redundant double-negation [of boolean value], together with
> > comparing boolean to char, provokes compilers to think the author
> > made logical mistakes here, although it works as expected.
> > Could you please try (if it's not automated build which you can't
> > modify) the following:
> > 
> 
> Agreed, the existing code seems wrong. The change below looks correct
> and fixes the problem. Feel free to add
> 
> Reviewed-and-tested-by: Guenter Roeck <linux@...ck-us.net>
> 
> to the real patch.
> 
> Thanks,
> Guenter
> 
> > --- a/arch/x86/platform/olpc/olpc-xo1-sci.c
> > +++ b/arch/x86/platform/olpc/olpc-xo1-sci.c
> > @@ -80,7 +80,7 @@ static void send_ebook_state(void)
> >   		return;
> >   	}
> > -	if (!!test_bit(SW_TABLET_MODE, ebook_switch_idev->sw) == state)
> > +	if (test_bit(SW_TABLET_MODE, ebook_switch_idev->sw) == !!state)
> >   		return; /* Nothing new to report. */
> >   	input_report_switch(ebook_switch_idev, SW_TABLET_MODE, state);
> > ---
> > 
> > We'd take it into the bitmap tree then. The series revealed
> > a fistful of existing code issues already :)

Would you like me to add your signed-off-by and apply, or you prefer
to send it out as a patch?

Thanks,
Yury