Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757928AbYHHRlg (ORCPT ); Fri, 8 Aug 2008 13:41:36 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752463AbYHHRl1 (ORCPT ); Fri, 8 Aug 2008 13:41:27 -0400 Received: from casper.infradead.org ([85.118.1.10]:53221 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751659AbYHHRl1 (ORCPT ); Fri, 8 Aug 2008 13:41:27 -0400 Subject: Re: [PATCH 0/2] printk vs rq->lock and xtime lock From: Peter Zijlstra To: Linus Torvalds Cc: Andrew Morton , mingo@elte.hu, tglx@linutronix.de, marcin.slusarz@gmail.com, linux-kernel@vger.kernel.org, David Miller , Steven Rostedt , Paul E McKenney In-Reply-To: References: <20080324122424.671168000@chello.nl> <1206382547.6437.131.camel@lappy> <20080324115738.85c72bb5.akpm@linux-foundation.org> <1218202249.8625.106.camel@twins> <1218215454.8625.133.camel@twins> Content-Type: text/plain Date: Fri, 08 Aug 2008 19:40:57 +0200 Message-Id: <1218217257.29098.2.camel@lappy.programming.kicks-ass.net> Mime-Version: 1.0 X-Mailer: Evolution 2.22.3.1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3464 Lines: 111 On Fri, 2008-08-08 at 10:25 -0700, Linus Torvalds wrote: > > On Fri, 8 Aug 2008, Peter Zijlstra wrote: > > > > Sure, but the RCU callback period is at least 3 jiffies and much longer > > when busy - I'm not sure how long before we force a grace period, we do > > that to avoid DoS, right Paul? > > I really don't think it matters. klogd is going to write the thing to > _disk_ (or network), and three jiffies really don't matter. If we can fill > the buffer in that kind of time, we're screwed for other reasons anyway. > > > So this version would have a much higher risk of overflowing the console > > buffer and making klogd miss bits. Then again, I really don't care about > > klogd at _all_, I've been running with the wakeup patched out for ages. > > Well, I'd care a _bit_ about klogd, but not enough to worry about a couple > of jiffies. We want to wake it up at some point, but... > > > Gah, the below doesn't boot - because I guess we start using rcu before > > its properly set up.. should I poke at it more? > > I'd certainly prefer this kind of approach. However, may I suggest: > > - doing the "waitqueue_active(&log_wait)" before even bothering to do the > RCU call. That, btw, will automatically mean that we wouldn't ever call > the RCU code before anything is initialized. > > - get rid of the "oops_in_progress" thing, since I think the whole point > of that was to avoid getting the lock recursively in the first place. > > - I'd worry about the "spin_lock_irqsave(&klogd_wakeup_state.lock)". What > if the printk happens from call_rcu()? This is exactly what we're > trying to get away from - having some parts of the kernel not able to > printk() because of subtle locking issues. > > For that last thing, maybe we can just make it a percpu thing and just > disable irq's? You're _so_ right! :-) This is much prettier and boots to boot. Signed-off-by: Peter Zijlstra --- diff --git a/kernel/printk.c b/kernel/printk.c index b51b156..10830d8 100644 --- a/kernel/printk.c +++ b/kernel/printk.c @@ -32,6 +32,7 @@ #include #include #include +#include #include @@ -982,10 +983,43 @@ int is_console_locked(void) return console_locked; } +void __wake_up_klogd(struct rcu_head *head); + +struct klogd_wakeup_state { + struct rcu_head head; + int pending; +}; + +DEFINE_PER_CPU(struct klogd_wakeup_state, kws); + +void __wake_up_klogd(struct rcu_head *head) +{ + unsigned long flags; + struct klogd_wakeup_state *kws = + container_of(head, struct klogd_wakeup_state, head); + + local_irq_save(flags); + BUG_ON(!kws->pending); + wake_up_interruptible(&log_wait); + kws->pending = 0; + local_irq_restore(flags); +} + void wake_up_klogd(void) { - if (!oops_in_progress && waitqueue_active(&log_wait)) - wake_up_interruptible(&log_wait); + unsigned long flags; + struct klogd_wakeup_state *kws; + + if (!waitqueue_active(&log_wait)) + return; + + local_irq_save(flags); + kws = &__get_cpu_var(kws); + if (!kws->pending) { + call_rcu(&kws->head, __wake_up_klogd); + kws->pending = 1; + } + local_irq_restore(flags); } /** -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/