From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751736AbbHSRTB (ORCPT ); Wed, 19 Aug 2015 13:19:01 -0400 Received: from mx1.redhat.com ([209.132.183.28]:58403 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750874AbbHSRTA (ORCPT ); Wed, 19 Aug 2015 13:19:00 -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] x86, bitops, variable_test_bit should return 1 not -1 on a match Date: Wed, 19 Aug 2015 13:18:54 -0400 Message-Id: <1440004734-24290-1-git-send-email-prarit@redhat.com> 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. 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 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 | 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 */ -- 1.7.9.3