From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S935250AbcI3Asu (ORCPT ); Thu, 29 Sep 2016 20:48:50 -0400 Received: from mail-pf0-f196.google.com ([209.85.192.196]:33869 "EHLO mail-pf0-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934998AbcI3Asl (ORCPT ); Thu, 29 Sep 2016 20:48:41 -0400 Date: Fri, 30 Sep 2016 09:48:32 +0900 From: Sergey Senozhatsky To: Petr Mladek Cc: Sergey Senozhatsky , Matt Fleming , Byungchul Park , Frederic Weisbecker , Jan Kara , Luca Abeni , Rik van Riel , Thomas Gleixner , Wanpeng Li , Yuyang Du , Mel Gorman , Mike Galbraith , Tejun Heo , Calvin Owens , linux-kernel@vger.kernel.org, Sergey Senozhatsky Subject: Re: [RFC 0/5] printk: Implement WARN_*DEFERRED() Message-ID: <20160930004832.GA547@swordfish> References: <1474992135-14777-1-git-send-email-pmladek@suse.com> <20160928011845.GA753@swordfish> <20160929112853.GC26796@pathway.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160929112853.GC26796@pathway.suse.cz> User-Agent: Mutt/1.7.0 (2016-08-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On (09/29/16 13:28), Petr Mladek wrote: > On Wed 2016-09-28 10:18:45, Sergey Senozhatsky wrote: > > On (09/27/16 18:02), Petr Mladek wrote: > > > The main trick is that we replace the per-CPU function pointer > > > by a preempt_count-like variable that could track the printk context. > > > > > > I know that Sergey has another ideas in this area. But I wanted to see > > > how this approach would look like. > > > > well, yes. I was looking at WARN_*_DEFERRED [1] for some time, and, I > > think, the maintenance cost of that solution is just too high: > > > > a) every existing WARN_* in sched/timekeeping/who knows where else > > must be evaluated to ensure that in can't be called from printk() > > path. if `false' - then the corresponding macro must be replaced > > with _DEFERRED flavor. > > > > b) any patch that adds new WARN_* usages must be additionally checked > > to ensure that each of new WARN_* macros cannot be called from printk > > path. if `false' -- the corresponding macro must be replaced with > > _DEFERRED flavor. > > > > c) any patch that refactors the code or moves some function calls around > > etc. must be additionally checked for any accidental WARN_* from printk > > path. even though if none of the patches added any new WARN_* to the code. > > > > b) apart from WARN_* there can be `accidental' pr_err/pr_debug/etc. not > > necessarily newly added (see 'c'). > > > > > > that's too much. > > > > it takes a lot of additional effort, because both reviewer and contributor > > must consider printk() internals. and, what's worse, if something goes > > unnoticed we end up having a printk() deadlock again. > > > > so I decided to address some of printk() issues in printk.c, not in > > kernel/time/timekeeping.c or kernel/sched/core.c or anywhere else. > > I see the point. well, just my 5 cents. > Your approach (alt buffer) adds some complexity to the printk code it does. the other thing is that there are several ways to deadlock printk(). alt_printk is addressing deadlocks that were caused by printk() recursion only. printk() acquire_lock(&foo) printk() acquire_lock(&foo) which is a sub-set of all of the printk() deadlock scenarios. all of the locks that printk() acquires can be taken outside of printk() path. for example, cat /proc/console locks the console_lock() for seq output. thus we can have something like console_unlock() // lock &sem->lock up() activate_task() WARN_ON() printk() console_trylock() // lock &sem->lock DEFERRED_WARN is a good thing; it's just quite hard to keep everything working, given that any of those "9 patches per hour" can break something with just one WARN_ON(). I assume that doing something like this #define WARN_ON(condition, format...) ({ \ printk_deferred_enter(); \ WARN(condition, ##format); \ printk_deferred_exit(); \ }) is less than exciting because WARN_ON from irq won't immediately print the backtrace anymore. thoughts? > but it allows to remove printk_deferred()/WARN_DEFERRED() and all > the risk of it. at some point we even can drop the entire deferred_printk() thing. but alt_printk needs some love and care first. > I am going to look closely on it. thanks. -ss