From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751341AbbJGX1U (ORCPT ); Wed, 7 Oct 2015 19:27:20 -0400 Received: from mx1.redhat.com ([209.132.183.28]:34187 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751020AbbJGX1T (ORCPT ); Wed, 7 Oct 2015 19:27:19 -0400 Message-ID: <5615AA55.5000203@redhat.com> Date: Wed, 07 Oct 2015 19:27:17 -0400 From: Prarit Bhargava User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 MIME-Version: 1.0 To: linux-kernel@vger.kernel.org CC: Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , x86@kernel.org Subject: Re: [PATCH v2] x86, bitops, variable_test_bit should return 1 not -1 on a match References: <1440440579-7535-1-git-send-email-prarit@redhat.com> In-Reply-To: <1440440579-7535-1-git-send-email-prarit@redhat.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org re-ping on this. Just making sure this wasn't dropped on the floor. P. On 08/24/2015 02:22 PM, Prarit Bhargava 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. > > It looks like the code never does, for example, (test_bit() == 1) so this > change should not have any impact. > > [v2]: hpa: Use setc, (Set if Carry, "Sets the byte in the operand to 1 if > the Carry Flag is set, otherwise sets the operand to 0.") instead of !! > > Cc: Thomas Gleixner > Cc: Ingo Molnar > Cc: "H. Peter Anvin" > Cc: x86@kernel.org > Cc: linux-kernel@vger.kernel.org > Signed-off-by: Prarit Bhargava > --- > arch/x86/include/asm/bitops.h | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h > index cfe3b95..c0bff87 100644 > --- a/arch/x86/include/asm/bitops.h > +++ b/arch/x86/include/asm/bitops.h > @@ -313,10 +313,10 @@ static __always_inline int constant_test_bit(long nr, const volatile unsigned lo > > static inline int variable_test_bit(long nr, volatile const unsigned long *addr) > { > - int oldbit; > + u8 oldbit; > > asm volatile("bt %2,%1\n\t" > - "sbb %0,%0" > + "setc %0" > : "=r" (oldbit) > : "m" (*(unsigned long *)addr), "Ir" (nr)); > >