On 08/21/2015 02:51 AM, Ingo Molnar wrote:
>
> * Prarit Bhargava <[email protected]> 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 <[email protected]>
>> Cc: Ingo Molnar <[email protected]>
>> Cc: "H. Peter Anvin" <[email protected]>
>> Cc: [email protected]
>> Cc: [email protected]
>> Signed-off-by: Prarit Bhargava <[email protected]>
>> ---
>> 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.
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