From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753046AbcHOSBh (ORCPT ); Mon, 15 Aug 2016 14:01:37 -0400 Received: from mail-pf0-f172.google.com ([209.85.192.172]:36724 "EHLO mail-pf0-f172.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752842AbcHOSBf (ORCPT ); Mon, 15 Aug 2016 14:01:35 -0400 Subject: Re: [V4 PATCH 2/2] mips/panic: Replace smp_send_stop() with kdump friendly version in panic path To: =?UTF-8?B?5rKz5ZCI6Iux5a6PIC8gS0FXQUnvvIxISURFSElSTw==?= , Dave Young References: <20160810080946.11028.97686.stgit@sysi4-13.yrl.intra.hitachi.co.jp> <20160810080950.11028.28000.stgit@sysi4-13.yrl.intra.hitachi.co.jp> <20160812031755.GB2983@dhcp-128-65.nay.redhat.com> <57ADD55D.1050003@mvista.com> <04EAB7311EE43145B2D3536183D1A84454CBBB00@GSjpTKYDCembx31.service.hitachi.net> Cc: Andrew Morton , "Eric W. Biederman" , Baoquan He , Ralf Baechle , "x86@kernel.org" , David Daney , Xunlei Pang , Aaro Koskinen , "kexec@lists.infradead.org" , "linux-kernel@vger.kernel.org" , HATAYAMA Daisuke , "linux-mips@linux-mips.org" , Masami Hiramatsu , "Steven J. Hill" , "xen-devel@lists.xenproject.org" , Daniel Walker , Vivek Goyal From: Corey Minyard Message-ID: <95d06858-6874-d795-dca5-ec33bf33d5da@mvista.com> Date: Mon, 15 Aug 2016 13:01:32 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.2.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 08/15/2016 12:06 PM, Corey Minyard wrote: > On 08/15/2016 06:35 AM, 河合英宏 / KAWAI，HIDEHIRO wrote: >> Hi Corey, >> >>> From: Corey Minyard [mailto:cminyard@mvista.com] >>> Sent: Friday, August 12, 2016 10:56 PM >>> I'll try to test this, but I have one comment inline... >> Thank you very much! >> >>> On 08/11/2016 10:17 PM, Dave Young wrote: >>>> On 08/10/16 at 05:09pm, Hidehiro Kawai wrote: >> [snip] >>>>> diff --git a/arch/mips/kernel/crash.c b/arch/mips/kernel/crash.c >>>>> index 610f0f3..1723b17 100644 >>>>> --- a/arch/mips/kernel/crash.c >>>>> +++ b/arch/mips/kernel/crash.c >>>>> @@ -47,9 +47,14 @@ static void crash_shutdown_secondary(void >>>>> *passed_regs) >>>>> >>>>> static void crash_kexec_prepare_cpus(void) >>>>> { >>>>> + static int cpus_stopped; >>>>> unsigned int msecs; >>>>> + unsigned int ncpus; >>>>> >>>>> - unsigned int ncpus = num_online_cpus() - 1;/* Excluding the >>>>> panic cpu */ >>>>> + if (cpus_stopped) >>>>> + return; >>> Wouldn't you want an atomic operation and some special handling here to >>> ensure that only one CPU does this? So if a CPU comes in here and >>> another CPU is already in the process stopping the CPUs it won't >>> result in a >>> deadlock. >> Because this function can be called only one panicking CPU, >> there is no problem. >> >> There are two paths which crash_kexec_prepare_cpus is called. >> >> Path 1 (panic path): >> panic() >> crash_smp_send_stop() >> crash_kexec_prepare_cpus() >> >> Path 2 (oops path): >> crash_kexec() >> __crash_kexec() >> machine_crash_shutdown() >> default_machine_crash_shutdown() // for MIPS >> crash_kexec_prepare_cpus() >> >> Here, panic() and crash_kexec() run exclusively via >> panic_cpu atomic variable. So we can use cpus_stopped as >> normal variable. > > Ok, if the code can only be entered once, what's the purpose of > cpus_stopped? > I guess that's what confused me. You are right, the panic_cpu atomic > should > keep this on a single CPU. Never mind, I see the path through panic() where that is required. My question below still remains, though. -corey > > Also, panic() will call panic_smp_self_stop() if it finds another CPU > has already > called panic, which will just spin with interrupts off by default. I > didn't see a > definition for it in MIPS, wouldn't it need to be overridden to avoid > a deadlock? > > -corey > >> >> Best regards, >> >> Hidehiro Kawai >> >