From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756423AbcCRFtl (ORCPT ); Fri, 18 Mar 2016 01:49:41 -0400 Received: from LGEAMRELO11.lge.com ([156.147.23.51]:48599 "EHLO lgeamrelo11.lge.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756239AbcCRFt0 (ORCPT ); Fri, 18 Mar 2016 01:49:26 -0400 X-Original-SENDERIP: 156.147.1.121 X-Original-MAILFROM: byungchul.park@lge.com X-Original-SENDERIP: 10.177.222.33 X-Original-MAILFROM: byungchul.park@lge.com Date: Fri, 18 Mar 2016 14:49:13 +0900 From: Byungchul Park To: Sergey Senozhatsky Cc: Sergey Senozhatsky , Jan Kara , Andrew Morton , Jan Kara , Petr Mladek , Tejun Heo , Tetsuo Handa , linux-kernel@vger.kernel.org Subject: Re: [RFC][PATCH v4 1/2] printk: Make printk() completely async Message-ID: <20160318054913.GN5220@X58A-UD3R> References: <1457964820-4642-1-git-send-email-sergey.senozhatsky@gmail.com> <1457964820-4642-2-git-send-email-sergey.senozhatsky@gmail.com> <20160315100323.GF17942@quack.suse.cz> <20160315140738.GA773@swordfish> <20160316053944.GJ5220@X58A-UD3R> <20160316065851.GC3217@swordfish> <20160316073007.GL5220@X58A-UD3R> <20160316075605.GE3217@swordfish> <20160316103431.GM5220@X58A-UD3R> <20160317003450.GA538@swordfish> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160317003450.GA538@swordfish> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Mar 17, 2016 at 09:34:50AM +0900, Sergey Senozhatsky wrote: > > I am curious about how you make the wake_up_process() call and I may want > > to talk about it at the next spin. Anyway, then we will lose the last > > message when "if (logbuf_cpu == this_cpu)" acts. Is it acceptible? > > yes, this is how it is. "BUG: recent printk recursion!" will be printed > instead of the message. I am not sure if it's the best way. For example, in the case suspecting rq->lock, we cannot inform about the rq's "lockup suspected" while a printk() is printing something, whatever it is, asynchronously. We can avoid the infinite recursion with the patch I attached below, even though the wake_up() and friends are used out of the section protected by logbuf_lock. > > IMHO it's not a good choice to use wake_up() and friend within a printk() > > since it can additionally cause another recursion. Of course, it does not > > happen if the condition (logbuf_cpu == this_cpu) acts. But I don't think > > it's good to rely on the condition with losing a message. Anyway I really > > really want to see your next spin and talk. > > the alternative is NOT significantly better. pending bit is checked in > IRQ, so one simply can do > > local_irq_save(); > while (xxx) printk(); > local_irq_restore(); > > and _in the worst case_ nothing will be printed to console until IRQ are Yes, you are right. But I am not sure yet. > I'd probably prefer to add wake_up_process() to vprintk_emit() and do it > under the logbuf lock. first, we don't suffer from disabled IRQs on current > CPU, second we have somewhat better chances to break printk() recursion > *in some cases*. I think the logbuf_cpu is not for it. It's a kind of last resort. It would be better to avoid using it if we can. And we can. > > This cannot happen. console_lock() cannot continue because the prior > > console_unlock() does not release console_sem.lock yet when > > wake_up_process() is called. Only a deadlock exists. And my patch solves > > the problem so that the deadlock cannot happen. > > ah, we lost in patches. I was talking about yet another patch > (you probably not aware of. you were not Cc'd. Sorry!) that > makes console_unlock() asynchronous: > > http://marc.info/?l=linux-kernel&m=145750373530161 I checked it now. Do you mean the wake_up_process() introduced in the new patch in console_unlock()? If so, I also think it does not make a deadlock, just can make a recursion in the worst case. I thought it was the wake_up_process() in up() which is eventually called from console_unlock(). A deadlock can happen with the wake_up_proces() in up(). :-) Thanks, Byungchul -----8<----- >>From 81f06a6f9c7f2e782267a2539c6c869d4214354c Mon Sep 17 00:00:00 2001 From: Byungchul Park Date: Fri, 18 Mar 2016 11:35:24 +0900 Subject: [PATCH] lib/spinlock_debug: Prevent a unnecessary recursive spin_dump() Printing "lockup suspected" for the same lock more than once is meaningless. Furtheremore, it can cause an infinite recursion if it's on the way printing something by printk(). Signed-off-by: Byungchul Park --- kernel/locking/spinlock_debug.c | 15 +++++++++++++-- 1 file changed, 13 insertions(+), 2 deletions(-) diff --git a/kernel/locking/spinlock_debug.c b/kernel/locking/spinlock_debug.c index fd24588..30559c6 100644 --- a/kernel/locking/spinlock_debug.c +++ b/kernel/locking/spinlock_debug.c @@ -138,14 +138,25 @@ static void __spin_lock_debug(raw_spinlock_t *lock) { u64 i; u64 loops = loops_per_jiffy * HZ; + static raw_spinlock_t *suspected_lock = NULL; for (i = 0; i < loops; i++) { if (arch_spin_trylock(&lock->raw_lock)) return; __delay(1); } - /* lockup suspected: */ - spin_dump(lock, "lockup suspected"); + + /* + * When we suspect a lockup, it's good enough to inform it once for + * the same lock. Otherwise it could cause an infinite recursion if + * it's within printk(). + */ + if (suspected_lock != lock) { + suspected_lock = lock; + /* lockup suspected: */ + spin_dump(lock, "lockup suspected"); + suspected_lock = NULL; + } #ifdef CONFIG_SMP trigger_all_cpu_backtrace(); #endif -- 1.9.1