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

[Prarit Bhargava <prarit@redhat.com>]

On 08/21/2015 02:51 AM, Ingo Molnar wrote:
> 
> * Prarit Bhargava <prarit@redhat.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@linutronix.de>
>> Cc: Ingo Molnar <mingo@redhat.com>
>> Cc: "H. Peter Anvin" <hpa@zytor.com>
>> Cc: x86@kernel.org
>> Cc: linux-kernel@vger.kernel.org
>> Signed-off-by: Prarit Bhargava <prarit@redhat.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?

I'm sorry Ingo, I think my description may have confused you.  I was debugging a
cpu hotplug issue[1] and did

printk("cpu %d cpu online status %d\n", cpu, cpu_online(cpu));

as a debug printk.  This printed out

cpu 3 cpu online status -1

which was really confusing.  That lead me down the rabbit hole of looking at the
sbb assembler instruction in variable_test_bit() to figure out why I was seeing -1.

P.

[1] The bug looks like it has to do with the system's firmware, not cpu hotplug.