From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752409AbbG2JWJ (ORCPT ); Wed, 29 Jul 2015 05:22:09 -0400 Received: from mail-wi0-f178.google.com ([209.85.212.178]:36595 "EHLO mail-wi0-f178.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751049AbbG2JWC (ORCPT ); Wed, 29 Jul 2015 05:22:02 -0400 Date: Wed, 29 Jul 2015 11:21:58 +0200 From: Michal Hocko To: =?utf-8?B?5rKz5ZCI6Iux5a6PIC8gS0FXQUnvvIxISURFSElSTw==?= Cc: Jonathan Corbet , Peter Zijlstra , Ingo Molnar , "Eric W. Biederman" , "H. Peter Anvin" , Andrew Morton , Thomas Gleixner , Vivek Goyal , "linux-doc@vger.kernel.org" , "x86@kernel.org" , "kexec@lists.infradead.org" , "linux-kernel@vger.kernel.org" , Ingo Molnar , =?utf-8?B?5bmz5p2+6ZuF5bezIC8gSElSQU1BVFXvvIxNQVNBTUk=?= Subject: Re: Re: [V2 PATCH 1/3] x86/panic: Fix re-entrance problem due to panic on NMI Message-ID: <20150729092157.GC15801@dhcp22.suse.cz> References: <20150727015850.4928.87717.stgit@softrs> <20150727015850.4928.50289.stgit@softrs> <20150727143405.GF11317@dhcp22.suse.cz> <55B6E2A3.8070004@hitachi.com> <04EAB7311EE43145B2D3536183D1A8445491D5E8@GSjpTKYDCembx31.service.hitachi.net> <20150729082329.GA15801@dhcp22.suse.cz> <04EAB7311EE43145B2D3536183D1A8445491DB5E@GSjpTKYDCembx31.service.hitachi.net> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <04EAB7311EE43145B2D3536183D1A8445491DB5E@GSjpTKYDCembx31.service.hitachi.net> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 29-07-15 09:09:18, 河合英宏 / KAWAI，HIDEHIRO wrote: > > From: Michal Hocko [mailto:mhocko@kernel.org] > > On Wed 29-07-15 05:48:47, 河合英宏 / KAWAI，HIDEHIRO wrote: > > > Hi, > > > > > > > From: linux-kernel-owner@vger.kernel.org [mailto:linux-kernel-owner@vger.kernel.org] On Behalf Of Hidehiro Kawai > > > > (2015/07/27 23:34), Michal Hocko wrote: > > > > > On Mon 27-07-15 10:58:50, Hidehiro Kawai wrote: > > > [...] > > > > > The check could be also relaxed a bit and nmi_panic would > > > > > return only if the ongoing panic is the current cpu when we really have > > > > > to return and allow the preempted panic to finish. > > > > > > > > It's reasonable. I'll do that in the next version. > > > > > > I noticed atomic_read() is insufficient. Please consider the following > > > scenario. > > > > > > CPU 1: call panic() in the normal context > > > CPU 0: call nmi_panic(), check the value of panic_cpu, then call panic() > > > CPU 1: set 1 to panic_cpu > > > CPU 0: fail to set 0 to panic_cpu, then do an infinite loop > > > CPU 1: call crash_kexec(), then call kdump_nmi_shootdown_cpus() > > > > > > At this point, since CPU 0 loops in NMI context, it never executes > > > the NMI handler registered by kdump_nmi_shootdown_cpus(). This means > > > that no register states are saved and no cleanups for VMX/SVM are > > > performed. > > > > Yes this is true but it is no different from the current state, isn't > > it? So if you want to handle that then it deserves a separate patch. > > It is certainly not harmful wrt. panic behavior. > > > > > So, we should still use atomic_cmpxchg() in nmi_panic() to > > > prevent other cpus from running panic routines. > > > > Not sure what you mean by that. > > I mean that we should use the same logic as my V2 patch like this: > > #define nmi_panic(fmt, ...) \ > do { \ > if (atomic_cmpxchg(&panic_cpu, -1, raw_smp_processor_id()) \ > == -1) \ > panic(fmt, ##__VA_ARGS__); \ > } while (0) This would allow to return from NMI too eagerly. When I was testing my previous approach (on 3.0 based kernel) I had basically the same thing (one NMI to process panic) and others to return. This led to a strange behavior when the NMI button triggered NMI on all (hundreds) CPUs. The crash kernel booted eventually but the log contained lockups when a CPU waited for an IPI to the CPU which was handling the NMI panic. Anyway, I do not thing this is really necessary to solve the panic reentrancy issue. If the missing saved state is a real problem then it should be handled separately - maybe it can be achieved without an IPI and directly from the panic context if we are in NMI. -- Michal Hocko SUSE Labs