Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753517Ab2BTPOw (ORCPT ); Mon, 20 Feb 2012 10:14:52 -0500 Received: from mx1.redhat.com ([209.132.183.28]:16227 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753215Ab2BTPOu (ORCPT ); Mon, 20 Feb 2012 10:14:50 -0500 Date: Mon, 20 Feb 2012 10:14:19 -0500 From: Don Zickus To: "Eric W. Biederman" Cc: Yinghai Lu , linux-kernel@vger.kernel.org, mingo@redhat.com, hpa@zytor.com, torvalds@linux-foundation.org, kexec@lists.infradead.org, vgoyal@redhat.com, akpm@linux-foundation.org, tglx@linutronix.de, mingo@elte.hu, linux-tip-commits@vger.kernel.org Subject: Re: [tip:x86/debug] x86/kdump: No need to disable ioapic/ lapic in crash path Message-ID: <20120220151419.GU9751@redhat.com> References: <20120216172735.GX9751@redhat.com> <20120216215603.GH9751@redhat.com> <20120217195430.GO9751@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2644 Lines: 61 On Fri, Feb 17, 2012 at 07:21:52PM -0800, Eric W. Biederman wrote: > Don Zickus writes: > > > On Fri, Feb 17, 2012 at 04:41:01AM -0800, Eric W. Biederman wrote: > >> > >> The fix with a guarantee of no more scope creep is to just disable the > >> nmi watchdog on the kexec on panic path. > >> > >> Don if you have time please figure out is needed to ignore nmi's and > >> possible record and/or report them while we boot, otherwise please cook > >> up a patch that just disables the nmi watchdog wherever we are sending > >> it from (the local apic or the ioapic). > > > > Can I keep things even simpler? The original problem was the deadlock > > with the ioapic lock. We fixed that by removing the call to > > disable_IO_APIC(). Can we just leave the disable_local_APIC calls in > > there for now? Is there any real harm? > > > All this rewrite is going to take time which will delay fixing a current > > problem with kexec on panic, the ioapic deadlock. > > Hmm. > > My apologies I just realized that we can not disable the nmi watchdog > safely in all cases. To avoid the deadlock we fundamentally can not > write to the io_apic, because the locks are the io_apic write path. > The nmi watchdog can be sourced from either the local apics or the > io_apics. To disable the nmi_watchdog we need at least potentially > to write io_apic. I am curious where you see the nmi watchdog being sourced from the ioapic? I thought I removed that code 3 or 4 releases ago. > > So it appears to me that the only reasonable and robust thing we can > do is to ignore nmis in the kexec on panic path. > > So it looks to me that the only path forward at this point is to fix > the other bug where an unexpected nmi will kill the kexec on panic boot. > > I just took a look at the code in /sbin/kexec and that code does not in > fact change the idt except when we switch to 16bit mode, which we > definitely do not do in the kexec on panic case. So it appears that we > don't need to coordinate an /sbin/kexec release with a kernel release to > ignore nmis. > > In fact it looks like we only need to fix the interrupt descriptors > loaded in machine_kexec_64.c and head64.c to ignore nmis. > > At which point we will have fixed two bugs and have a much more reliable > kexec on panic implementation. Ok. I'll talk with Vivek about how the can be implemented. Cheers. Don -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/