From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932162AbcHVEPP (ORCPT ); Mon, 22 Aug 2016 00:15:15 -0400 Received: from mail-pf0-f196.google.com ([209.85.192.196]:33126 "EHLO mail-pf0-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752780AbcHVEPO (ORCPT ); Mon, 22 Aug 2016 00:15:14 -0400 Date: Mon, 22 Aug 2016 13:15:20 +0900 From: Sergey Senozhatsky To: Jan Kara , Petr Mladek Cc: Sergey Senozhatsky , Sergey Senozhatsky , Viresh Kumar , Andrew Morton , 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: <20160822041520.GA511@swordfish> References: <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> <20160820052430.GA695@swordfish> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160820052430.GA695@swordfish> 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 Hello, On (08/20/16 14:24), Sergey Senozhatsky wrote: > 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. so we probably can try something like this --- diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 2add7c5..b23f919 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2014,6 +2014,7 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags) * set_current_state() the waiting thread does. */ smp_mb__before_spinlock(); + printk_deferred_enter(); raw_spin_lock_irqsave(&p->pi_lock, flags); if (!(p->state & state)) goto out; @@ -2073,6 +2074,7 @@ stat: ttwu_stat(p, cpu, wake_flags); out: raw_spin_unlock_irqrestore(&p->pi_lock, flags); + printk_deferred_exit(); return success; } --- since we are basically interested in wake_up_process() only from printk() POV. not sure how acceptable 2 * preempt_count and 2 * per-CPU writes for every try_to_wake_up(). the other thing I just thought of is doing something as follows !!!not tested, will not compile, just an idea!!! --- diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 6e260a0..bb8d719 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -1789,6 +1789,7 @@ asmlinkage int vprintk_emit(int facility, int level, printk_delay(); local_irq_save(flags); + printk_nmi_enter(); this_cpu = smp_processor_id(); /* @@ -1804,6 +1805,7 @@ asmlinkage int vprintk_emit(int facility, int level, */ if (!oops_in_progress && !lockdep_recursing(current)) { recursion_bug = true; + printk_nmi_exit(); local_irq_restore(flags); return 0; } @@ -1920,6 +1922,7 @@ asmlinkage int vprintk_emit(int facility, int level, logbuf_cpu = UINT_MAX; raw_spin_unlock(&logbuf_lock); lockdep_on(); + printk_nmi_exit(); local_irq_restore(flags); /* If called from the scheduler, we can not call up(). */ --- so may be we will not blow up in case of spin_dump() on logbuf_lock. well, if logbuf_lock was corrupted then nothing will help us -- it's over. but if raw_spin_lock(&logbuf_lock), for instance, spin_dump()-s because logbuf_lock was not released after `loops_per_jiffy * HZ' then deferred printing may help. probably we can make NMI printk_func to be more general way of using an alternative-printk buffer and, for example, rename API it to printk_alternative_enter() and printk_alternative_exit(), not to confuse anyone. (yes, NMIs are not available on every platform) and even more. we can start dump_stack() from recursion detection path to that alternative printk-buffer, which is a bit more helpful than "BUG: recent printk recursion!" !!!not tested, will not compile, just an idea!!! --- diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 6e260a0..ebce39a 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -1789,6 +1789,7 @@ asmlinkage int vprintk_emit(int facility, int level, printk_delay(); local_irq_save(flags); + printk_nmi_enter(); this_cpu = smp_processor_id(); /* @@ -1804,6 +1805,8 @@ asmlinkage int vprintk_emit(int facility, int level, */ if (!oops_in_progress && !lockdep_recursing(current)) { recursion_bug = true; + WARN_ON(1); /* <<< dump_stack to alternative buffer */ + printk_nmi_exit(); local_irq_restore(flags); return 0; } @@ -1816,14 +1819,8 @@ asmlinkage int vprintk_emit(int facility, int level, logbuf_cpu = this_cpu; if (unlikely(recursion_bug)) { - static const char recursion_msg[] = - "BUG: recent printk recursion!"; - recursion_bug = false; - /* emit KERN_CRIT message */ - printed_len += log_store(0, 2, LOG_PREFIX|LOG_NEWLINE, 0, - NULL, 0, recursion_msg, - strlen(recursion_msg)); + printk_nmi_flush(); } nmi_message_lost = get_nmi_message_lost(); @@ -1920,6 +1917,7 @@ asmlinkage int vprintk_emit(int facility, int level, logbuf_cpu = UINT_MAX; raw_spin_unlock(&logbuf_lock); lockdep_on(); + printk_nmi_exit(); local_irq_restore(flags); /* If called from the scheduler, we can not call up(). */ --- just some quick thoughts. -ss