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 06:49:46 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Alexander Lobakin <alexandr.lobakin@...el.com>
Cc:     Arnd Bergmann <arnd@...db.de>, Yury Norov <yury.norov@...il.com>,
        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 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 :)
> 
>>
>> Bisect log attached.
>>
>> Guenter
>>
>> ---
>> # bad: [4662b7adea50bb62e993a67f611f3be625d3df0d] Add linux-next specific files for 20220713
>> # good: [32346491ddf24599decca06190ebca03ff9de7f8] Linux 5.19-rc6
>> git bisect start 'HEAD' 'v5.19-rc6'
>> # good: [8b7e002d8bc6e17c94092d25e7261db4e6e5f2cc] Merge branch 'drm-next' of git://git.freedesktop.org/git/drm/drm.git
>> git bisect good 8b7e002d8bc6e17c94092d25e7261db4e6e5f2cc
>> # good: [07f6d21d6e33c1e28e24ae84e9d26e4e7d4853f5] Merge branch 'next' of git://git.kernel.org/pub/scm/linux/kernel/git/kvms390/linux.git
>> git bisect good 07f6d21d6e33c1e28e24ae84e9d26e4e7d4853f5
>> # good: [5ff085e5d4f6700e03635d5e700f52163a6dc2a7] Merge branch 'staging-next' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/staging.git
>> git bisect good 5ff085e5d4f6700e03635d5e700f52163a6dc2a7
>> # good: [eb9e3fdbdd8b61ef0f4bee23259fe6ab69e463ab] Merge branch 'master' of git://git.kernel.org/pub/scm/linux/kernel/git/crng/random.git
>> git bisect good eb9e3fdbdd8b61ef0f4bee23259fe6ab69e463ab
>> # good: [9f2183cd961e5ddb7954eafb6bb01a495c6a9c7b] hexagon/mm: enable ARCH_HAS_VM_GET_PAGE_PROT
>> git bisect good 9f2183cd961e5ddb7954eafb6bb01a495c6a9c7b
>> # bad: [e878aa5faf9ac8c0b5d0c3f293389c194c250fff] Merge branch 'mm-nonmm-stable' of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
>> git bisect bad e878aa5faf9ac8c0b5d0c3f293389c194c250fff
>> # good: [cf95d50205f62c4f5f538676def847292cf39fa9] fs: don't call ->writepage from __mpage_writepage
>> git bisect good cf95d50205f62c4f5f538676def847292cf39fa9
>> # good: [5103cbfd92d3587713476f94f9485b96e02f0146] Merge branch 'for-next/execve' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git
>> git bisect good 5103cbfd92d3587713476f94f9485b96e02f0146
>> # good: [ee56c3e8eec166f4e4a2ca842b7804d14f3a0208] Merge branch 'master' into mm-nonmm-stable
>> git bisect good ee56c3e8eec166f4e4a2ca842b7804d14f3a0208
>> # bad: [dc34d5036692c614eef23c1130ee42a201c316bf] lib: test_bitmap: add compile-time optimization/evaluations assertions
>> git bisect bad dc34d5036692c614eef23c1130ee42a201c316bf
>> # good: [bb7379bfa680bd48b468e856475778db2ad866c1] bitops: define const_*() versions of the non-atomics
>> git bisect good bb7379bfa680bd48b468e856475778db2ad866c1
>> # bad: [b03fc1173c0c2bb8fad61902a862985cecdc4b1b] bitops: let optimize out non-atomic bitops on compile-time constants
>> git bisect bad b03fc1173c0c2bb8fad61902a862985cecdc4b1b
>> # good: [e69eb9c460f128b71c6b995d75a05244e4b6cc3e] bitops: wrap non-atomic bitops with a transparent macro
>> git bisect good e69eb9c460f128b71c6b995d75a05244e4b6cc3e
>> # first bad commit: [b03fc1173c0c2bb8fad61902a862985cecdc4b1b] bitops: let optimize out non-atomic bitops on compile-time constants
> 
> Thanks,
> Olek