From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755199AbbHXSXH (ORCPT ); Mon, 24 Aug 2015 14:23:07 -0400 Received: from mx1.redhat.com ([209.132.183.28]:54132 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755140AbbHXSXD (ORCPT ); Mon, 24 Aug 2015 14:23:03 -0400 From: Prarit Bhargava To: linux-kernel@vger.kernel.org Cc: Prarit Bhargava , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , x86@kernel.org Subject: [PATCH v2] x86, bitops, variable_test_bit should return 1 not -1 on a match Date: Mon, 24 Aug 2015 14:22:59 -0400 Message-Id: <1440440579-7535-1-git-send-email-prarit@redhat.com> In-Reply-To: References: Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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)); -- 1.7.9.3