From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753694AbbHULuJ (ORCPT ); Fri, 21 Aug 2015 07:50:09 -0400 Received: from mx1.redhat.com ([209.132.183.28]:35691 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752960AbbHULuH (ORCPT ); Fri, 21 Aug 2015 07:50:07 -0400 Message-ID: <55D7106D.3040904@redhat.com> Date: Fri, 21 Aug 2015 07:50:05 -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: Ingo Molnar CC: linux-kernel@vger.kernel.org, Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , x86@kernel.org, Linus Torvalds , Andrew Morton , Peter Zijlstra Subject: Re: [PATCH] x86, bitops, variable_test_bit should return 1 not -1 on a match References: <1440004734-24290-1-git-send-email-prarit@redhat.com> <20150821065103.GA4541@gmail.com> In-Reply-To: <20150821065103.GA4541@gmail.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 On 08/21/2015 02:51 AM, Ingo Molnar wrote: > > * 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. 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 */ > > 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? I'm sorry Ingo, I think my description may have confused you. I was debugging a cpu hotplug issue[1] and did printk("cpu %d cpu online status %d\n", cpu, cpu_online(cpu)); as a debug printk. This printed out cpu 3 cpu online status -1 which was really confusing. That lead me down the rabbit hole of looking at the sbb assembler instruction in variable_test_bit() to figure out why I was seeing -1. P. [1] The bug looks like it has to do with the system's firmware, not cpu hotplug.