From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753163AbcHORG4 (ORCPT ); Mon, 15 Aug 2016 13:06:56 -0400 Received: from mail-pf0-f171.google.com ([209.85.192.171]:36013 "EHLO mail-pf0-f171.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752824AbcHORGy (ORCPT ); Mon, 15 Aug 2016 13:06:54 -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: Date: Mon, 15 Aug 2016 12:06:50 -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: <04EAB7311EE43145B2D3536183D1A84454CBBB00@GSjpTKYDCembx31.service.hitachi.net> 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 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. 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 >