From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756639AbaE2IJz (ORCPT ); Thu, 29 May 2014 04:09:55 -0400 Received: from cantor2.suse.de ([195.135.220.15]:43081 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756330AbaE2IJw (ORCPT ); Thu, 29 May 2014 04:09:52 -0400 Date: Thu, 29 May 2014 10:09:48 +0200 (CEST) From: Jiri Kosina To: Frederic Weisbecker cc: Petr Mladek , Andrew Morton , Steven Rostedt , Dave Anderson , "Paul E. McKenney" , Kay Sievers , Michal Hocko , Jan Kara , linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH 00/11] printk: safe printing in NMI context In-Reply-To: <20140529000909.GC6507@localhost.localdomain> Message-ID: References: <1399626665-29817-1-git-send-email-pmladek@suse.cz> <20140529000909.GC6507@localhost.localdomain> User-Agent: Alpine 2.00 (LNX 1167 2008-08-23) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 29 May 2014, Frederic Weisbecker wrote: > > I am rather surprised that this patchset hasn't received a single review > > comment for 3 weeks. > > > > Let me point out that the issues Petr is talking about in the cover letter > > are real -- we've actually seen the lockups triggered by RCU stall > > detector trying to dump stacks on all CPUs, and hard-locking machine up > > while doing so. > > > > So this really needs to be solved. > > The lack of review may be partly due to a not very appealing changestat > on an old codebase that is already unpopular: > > Documentation/kernel-parameters.txt | 19 +- > kernel/printk/printk.c | 1218 +++++++++++++++++++++++++---------- > 2 files changed, 878 insertions(+), 359 deletions(-) > > > Your patches look clean and pretty nice actually. They must be seriously > considered if we want to keep the current locked ring buffer design and > extend it to multiple per context buffers. But I wonder if it's worth to > continue that way with the printk ancient design. > > If it takes more than 1000 line changes (including 500 added) to make it > finally work correctly with NMIs by working around its fundamental > flaws, shouldn't we rather redesign it to use a lockless ring buffer > like ftrace or perf ones? Yeah, printk() has grown over years to a stinking pile of you-know-what, no argument to that. I also agree that performing a massive rewrite, which will make it use a lockless buffer, and therefore ultimately solve all its problems (scheduler deadlocks, NMI deadlocks, xtime_lock deadlocks) at once, is necessary in the long run. On the other hand, I am completely sure that the diffstat for such rewrite is going to be much more scary :) This is not adding fancy features to printk(), where we really should be saying no; horrible commits like 7ff9554bb5 is exactly something that should be pushed against *heavily*. But bugfixes for hard machine lockups are a completely different story to me (until we have a whole new printk() buffer handling implementation). -- Jiri Kosina SUSE Labs