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

[Ingo Molnar <mingo@kernel.org>]

* 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?

Thanks,

	Ingo