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

Guenter Roeck
2022-07-15T13:49:46Z

On 7/15/22 06:26, Alexander Lobakin wrote:
> From: Guenter Roeck <linux@roeck-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@arm.com>
>>> Signed-off-by: Alexander Lobakin <alexandr.lobakin@intel.com>
>>> Reviewed-by: Marco Elver <elver@google.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@roeck-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