From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752449AbcHTFZL (ORCPT ); Sat, 20 Aug 2016 01:25:11 -0400 Received: from mail-pf0-f195.google.com ([209.85.192.195]:33948 "EHLO mail-pf0-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751161AbcHTFZK (ORCPT ); Sat, 20 Aug 2016 01:25:10 -0400 Date: Sat, 20 Aug 2016 14:24:30 +0900 From: Sergey Senozhatsky To: Jan Kara Cc: Petr Mladek , Sergey Senozhatsky , Viresh Kumar , Andrew Morton , Sergey Senozhatsky , Jan Kara , Tejun Heo , Tetsuo Handa , "linux-kernel@vger.kernel.org" , Byungchul Park , vlevenetz@mm-sol.com, Greg Kroah-Hartman Subject: Re: [PATCH v10 1/2] printk: Make printk() completely async Message-ID: <20160820052430.GA695@swordfish> References: <20160406082758.GA3554@quack.suse.cz> <20160812094447.GD7339@pathway.suse.cz> <20160818022712.GB500@swordfish> <20160818093329.GL13300@pathway.suse.cz> <20160818095144.GA425@swordfish> <20160818105629.GE26194@pathway.suse.cz> <20160819063236.GA584@swordfish> <20160819095455.GR13300@pathway.suse.cz> <20160819190007.GA8275@quack2.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160819190007.GA8275@quack2.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 (08/19/16 21:00), Jan Kara wrote: > > > depending on .config BUG() may never return back -- passing control > > > to do_exit(), so printk_deferred_exit() won't be executed. thus we > > > probably need to have a per-cpu variable that would indicate that > > > we are in deferred_bug. hm... but do we really need deferred BUG() > > > in the first place? > > > > Good question. I am not aware of any BUG_ON() that would be called from > > wake_up_process() but it is hard to check everything. > > > > A conservative approach would be to force synchronous printk from > > BUG_ON(). > > Just a quick thought: Cannot we just do printk_deferred_enter() when we are > about to call into the scheduler from printk code and printk_deferred_exit() > when leaving it? That would look like the least error-prone way how > handling this kind of recursion... interesting idea. printk_deferred_enter() increments preempt count, so there may be additional obstacles and, as a result, ad-hocs, that scheduler people will sincerely hate. need to think more. > OTOH there's also the other possible direction for the recursion when we > are in the scheduler, holding some scheduler locks, decide to WARN which > enters printk, that ends up calling wake_up_process() which deadlocks > on scheduler locks... I don't see how to handle this type of recursion > inside the printk code itself easily and so far the answer was - use > printk_deferred() in the scheduler and don't use WARN... the recursion detection is really tricky, yes. it seems (and I haven't thought of it good enough) to be a bit simpler when we operate in async printk mode, because we remove this uncontrollable console_unlock(). so we can do something like this: vprintk_emit(....) { local_irq_save(); if (this_cpu_read(in_printk)) { log_store(BUG: printk recursion!"); goto out; } this_cpu_write(in_printk) = 1; raw_spin_lock(&logbuf_lock); log_store(); raw_spin_unlock(&logbuf_lock); if (!in_sched) { if (console_loglevel != CONSOLE_LOGLEVEL_MOTORMOUTH && can_printk_async()) { printk_kthread_need_flush_console = true; wake_up_process(printk_kthread); } } this_cpu_write(in_printk) = 0; out: local_irq_restore(); } async printk mode from this point of view is sort of atomic. we can even set different values of per-CPU `in_printk' on various stages of printk, which will permit to have better recursion handling. for example, if we recurse from raw_spin_unlock(&logbuf_lock) then we must re-init logbuf_lock, because it's 99% corrupted... and so on. but I haven't really thought of it yet. it obviously doesn't work for sync printk mode. > Hum, maybe we could add lockdep annotation to a WARN_ON and BUG_ON macros so > that it would grab and release console_sem (even if the condition is false). > That way we'd get lockdep splats for all the possible WARN_ON and BUG_ON > calls that could deadlock. hm. -ss