* Prarit Bhargava <pra...@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 <t...@linutronix.de>
> Cc: Ingo Molnar <mi...@redhat.com>
> Cc: "H. Peter Anvin" <h...@zytor.com>
> Cc: x...@kernel.org
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Prarit Bhargava <pra...@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
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