From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-x234.google.com (mail-wm0-x234.google.com [IPv6:2a00:1450:400c:c09::234]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 7CFC71A06BA for ; Wed, 2 Mar 2016 01:04:26 +1100 (AEDT) Received: by mail-wm0-x234.google.com with SMTP id p65so34925617wmp.0 for ; Tue, 01 Mar 2016 06:04:26 -0800 (PST) Subject: Re: [PATCH v3 4/4] printk/nmi: Increase the size of NMI buffer and make it configurable To: Petr Mladek References: <1449667265-17525-1-git-send-email-pmladek@suse.com> <1449667265-17525-5-git-send-email-pmladek@suse.com> <20151211124159.GB3729@pathway.suse.cz> <20151211145725.b0e81bb4bb18fcd72ef5f557@linux-foundation.org> <20151211232113.GZ8644@n2100.arm.linux.org.uk> <5673DD60.7080302@linaro.org> <20151218145207.GK3729@pathway.suse.cz> <56743BA0.1030409@linaro.org> Cc: Jiri Kosina , Russell King - ARM Linux , Andrew Morton , Geert Uytterhoeven , Peter Zijlstra , Steven Rostedt , Ingo Molnar , Thomas Gleixner , "linux-kernel@vger.kernel.org" , the arch/x86 maintainers , "linux-arm-kernel@lists.infradead.org" , "adi-buildroot-devel@lists.sourceforge.net" , Cris , Linux MIPS Mailing List , "linuxppc-dev@lists.ozlabs.org" , linux-s390 , Linux-sh list , sparclinux From: Daniel Thompson Message-ID: <56D5A164.3000000@linaro.org> Date: Tue, 1 Mar 2016 14:04:20 +0000 MIME-Version: 1.0 In-Reply-To: <56743BA0.1030409@linaro.org> Content-Type: text/plain; charset=windows-1252; format=flowed List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On 18/12/15 17:00, Daniel Thompson wrote: >>> The MCE handlers should only call printk() when they decide to panic >>> and *after* busting the spinlocks. At this point deferring printk() >>> until it is safe is not very helpful. >>> >>> When we bust the spinlocks we should probably restore the normal >>> printk() function to give best chance of the failure messages making >>> it out. >> >> The problem is that we do not know what locks need to be busted. There >> are too many consoles and too many locks involved. Also busting locks >> open another can of worms. > > Yes, I agree that busting the spinlocks doesn't avoid all risk of deadlock. > > Probably I've been placing too much weight on the importance of getting > messages out when dying. You're right that surviving far enough through > a panic to trigger kdump or reset is equally (or more) important in many > scenarios than getting a failure message out. > > However on a system with nothing but "while(1) {}" hooked up to panic() > then its worth risking a lock up. In this case restoring normal printk() > behavior and dumping the NMI buffers would be worthwhile. An a (much) later thread[1] Andrew Morton described this comment as non-committal. Sorry for that. I don't object to the overall approach taken by Petr, merely that I think there are cases where the current patchset doesn't put in quite enough effort to issue messages. Panic triggered during NMI is the only case I can think of and that, I think, could be addressed by adding an extra call to printk_nmi_flush() during panic(). It should probably only cover cases where we *don't* call kdump and the panic handler doesn't restart the machine... so just after the pr_emerg("...end kernel panic") would be OK for me. Daniel. [1]: http://thread.gmane.org/gmane.linux.ports.arm.kernel/482845