From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752743AbbCWNvA (ORCPT ); Mon, 23 Mar 2015 09:51:00 -0400 Received: from mail-wi0-f173.google.com ([209.85.212.173]:37115 "EHLO mail-wi0-f173.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752631AbbCWNuv (ORCPT ); Mon, 23 Mar 2015 09:50:51 -0400 Date: Mon, 23 Mar 2015 14:50:46 +0100 From: Ingo Molnar To: Vivek Goyal Cc: Baoquan He , =?utf-8?B?IkhhdGF5YW1hLCBEYWlzdWtlL+eVkeWxsSDlpKfovJQi?= , ebiederm@xmission.com, masami.hiramatsu.pt@hitachi.com, hidehiro.kawai.ez@hitachi.com, linux-kernel@vger.kernel.org, kexec@lists.infradead.org, akpm@linux-foundation.org, mingo@redhat.com, bp@suse.de Subject: Re: [PATCH v2] kernel/panic/kexec: fix "crash_kexec_post_notifiers" option issue in oops path Message-ID: <20150323135046.GA25012@gmail.com> References: <54F9D645.2050008@jp.fujitsu.com> <20150323034752.GD2068@dhcp-16-105.nay.redhat.com> <20150323071943.GA22765@gmail.com> <20150323133710.GA3172@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20150323133710.GA3172@redhat.com> 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 * Vivek Goyal wrote: > On Mon, Mar 23, 2015 at 08:19:43AM +0100, Ingo Molnar wrote: > > > > * Baoquan He wrote: > > > > > CC more people ... > > > > > > On 03/07/15 at 01:31am, "Hatayama, Daisuke/畑山 大輔" wrote: > > > > The commit f06e5153f4ae2e2f3b0300f0e260e40cb7fefd45 introduced > > > > "crash_kexec_post_notifiers" kernel boot option, which toggles > > > > wheather panic() calls crash_kexec() before panic_notifiers and dump > > > > kmsg or after. > > > > > > > > The problem is that the commit overlooks panic_on_oops kernel boot > > > > option. If it is enabled, crash_kexec() is called directly without > > > > going through panic() in oops path. > > > > > > > > To fix this issue, this patch adds a check to > > > > "crash_kexec_post_notifiers" in the condition of kexec_should_crash(). > > > > > > > > Also, put a comment in kexec_should_crash() to explain not obvious > > > > things on this patch. > > > > > > > > Signed-off-by: HATAYAMA Daisuke > > > > Acked-by: Baoquan He > > > > Tested-by: Hidehiro Kawai > > > > Reviewed-by: Masami Hiramatsu > > > > --- > > > > include/linux/kernel.h | 3 +++ > > > > kernel/kexec.c | 11 +++++++++++ > > > > kernel/panic.c | 2 +- > > > > 3 files changed, 15 insertions(+), 1 deletion(-) > > > > This is hack upon hack, but why was this crap merged in the first > > place? > > > > I see two problems just by cursory review: > > > > 1) > > > > Firstly, the real bug in: > > > > f06e5153f4ae ("kernel/panic.c: add "crash_kexec_post_notifiers" option for kdump after panic_notifers") > > > > Was that crash_kexec() was called unconditionally after notifiers were > > called, which should be fixed via the simple patch below (untested). > > Looks much simpler than your fix. > > > > Hi Ingo, > > Agreed. Your patch looks good. In case you want that simpler fix and need my SOB: Signed-off-by: Ingo Molnar (but I have not tested it.) > > Secondly, and more importantly, the whole premise of commit > > f06e5153f4ae is broken IMHO: > > > > "This can help rare situations where kdump fails because of unstable > > crashed kernel or hardware failure (memory corruption on critical > > data/code)" > > > > wtf? > > > > If the kernel crashed due to a kernel crash, then the kernel booting > > up in whatever hardware state should be able to do a clean bootup. The > > fix for those 'rare situations' should be to fix the real bug (for > > example by making hardware driver init (or deinit) sequences more > > robust), not to paper it over by ordering around crash-time sequences > > ... > > > > If it crashed due to some hardware failure, there's literally an > > infinite amount of failure modes that may or may not be impacted by > > kexec crash-time handling ordering. We don't want to put a zillion > > such flags into the kernel proper just to allow the perturbation of > > the kernel. > > I think one of the motivations behind this patch was call to kmsg_dump(). > Some vendors have been wanting to have the capability to save kernel logs > to some NVRAM before transition to second kernel happens. Their argument > is that kdump does not succeed all the time and if kdump does not succeed > then atleast they have something to work with (kernel logs retrieved > from pstore interface). Doesn't pstore attach itself to printk itself? AFAICS it does: fs/pstore/platform.c: register_console(&pstore_console); so the printk log leading up to and including the crash should be available, regardless of this patch. What am I missing? > Not that I agree fully with this as problem might happen while we > try to run panic_notifiers or kmsg_dump hooks and never transition > into kdump kernel. btw., this is the big problem with 'notifiers' in general: they are opaque with barely any semantics defined, and a source of constant confusion. > And it has been literally years since some developers have been > pushing for allowing to run panic notifiers before crash_kexec(). > Eric Biederman has been pushing back saying it reduces the > reliability of kdump operation so this is not acceptable. So what do those notifiers do? Thanks, Ingo