linux-kernel - Re: [PATCH] x86, bitops, variable_test_bit should return 1 not -1 on a match

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Message-ID: <E661604A-0C9E-4C4F-84C3-BDEAEE2E18F3@zytor.com>
Date:	Fri, 21 Aug 2015 01:08:28 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ingo Molnar <mingo@...nel.org>, Prarit Bhargava <prarit@...hat.com>
CC:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] x86, bitops, variable_test_bit should return 1 not -1 on a match

Wrong fix, though.  Instead we should change it to use the set instruction, which would also make it easier to use the CC_SET/CC_OUT proposed macros to use assembly out in the future.

The downside with set is that it only sets a single byte, the upside is that it always outputs 0 or 1, and apparently if the output variable is your bool gcc can use that for optimization.


On August 20, 2015 11:51:03 PM PDT, Ingo Molnar <mingo@...nel.org> wrote:
>
>* Prarit Bhargava <prarit@...hat.com> wrote:
>
>> This issue was noticed while debugging a CPU hotplug issue.  On x86
>> with (NR_CPUS > 1) the cpu_online() define is cpumask_test_cpu().
>> cpumask_test_cpu() should return 1 if the cpu is set in cpumask and
>> 0 otherwise.
>> 
>> However, cpumask_test_cpu() returns -1 if the cpu in the cpumask is
>> set and 0 otherwise.  This happens because cpumask_test_cpu() calls
>> test_bit() which is a define that will call variable_test_bit().
>> 
>> variable_test_bit() calls the assembler instruction sbb (Subtract
>> with Borrow, " Subtracts the source from the destination, and
>subtracts 1
>> extra if the Carry Flag is set. Results are returned in "dest".)
>> 
>> A bit match results in -1 being returned from variable_test_bit() if
>a
>> match occurs, not 1 as the function is supposed to.  This can be
>easily
>> resolved by adding a "!!" to force 0 or 1 as a return.
>> 
>> It looks like the code never does, for example, (test_bit() == 1) so
>this
>> change should not have any impact.
>> 
>> Cc: Thomas Gleixner <tglx@...utronix.de>
>> Cc: Ingo Molnar <mingo@...hat.com>
>> Cc: "H. Peter Anvin" <hpa@...or.com>
>> Cc: x86@...nel.org
>> Cc: linux-kernel@...r.kernel.org
>> Signed-off-by: Prarit Bhargava <prarit@...hat.com>
>> ---
>>  arch/x86/include/asm/bitops.h |    2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/arch/x86/include/asm/bitops.h
>b/arch/x86/include/asm/bitops.h
>> index cfe3b95..a87a5fb 100644
>> --- a/arch/x86/include/asm/bitops.h
>> +++ b/arch/x86/include/asm/bitops.h
>> @@ -320,7 +320,7 @@ static inline int variable_test_bit(long nr,
>volatile const unsigned long *addr)
>>  		     : "=r" (oldbit)
>>  		     : "m" (*(unsigned long *)addr), "Ir" (nr));
>>  
>> -	return oldbit;
>> +	return !!oldbit;
>>  }
>>  
>>  #if 0 /* Fool kernel-doc since it doesn't do macros yet */
>
>Ok, I think this is a good fix to improve the robustness of this
>primitive, unless 
>someone objects.
>
>I tried to find the CPU hotplug code that broke with cpu_online()
>returning -1 but 
>failed - all current mainline usage sites seem to be testing for
>nonzero in one 
>way or another. Could you please point it out?
>
>Thanks,
>
>	Ingo

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