Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759578AbYHHUa7 (ORCPT ); Fri, 8 Aug 2008 16:30:59 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753201AbYHHUaw (ORCPT ); Fri, 8 Aug 2008 16:30:52 -0400 Received: from e33.co.us.ibm.com ([32.97.110.151]:52915 "EHLO e33.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753033AbYHHUav (ORCPT ); Fri, 8 Aug 2008 16:30:51 -0400 Date: Fri, 8 Aug 2008 13:30:49 -0700 From: "Paul E. McKenney" To: Andrew Morton Cc: Peter Zijlstra , torvalds@linux-foundation.org, mingo@elte.hu, tglx@linutronix.de, marcin.slusarz@gmail.com, linux-kernel@vger.kernel.org, davem@davemloft.net, rostedt@goodmis.org Subject: Re: [PATCH] printk: robustify printk Message-ID: <20080808203049.GG6760@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1206382547.6437.131.camel@lappy> <20080324115738.85c72bb5.akpm@linux-foundation.org> <1218202249.8625.106.camel@twins> <1218215454.8625.133.camel@twins> <1218217257.29098.2.camel@lappy.programming.kicks-ass.net> <1218219269.29098.5.camel@lappy.programming.kicks-ass.net> <20080808121428.646a8b3c.akpm@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20080808121428.646a8b3c.akpm@linux-foundation.org> User-Agent: Mutt/1.5.15+20070412 (2007-04-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1847 Lines: 50 On Fri, Aug 08, 2008 at 12:14:28PM -0700, Andrew Morton wrote: > On Fri, 08 Aug 2008 20:14:28 +0200 > Peter Zijlstra wrote: > > > 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) { > > + kws->pending = 1; > > + call_rcu(&kws->head, __wake_up_klogd); > > + } > > + local_irq_restore(flags); > > } > > Note that kernel/rcupreempt.c's flavour of call_rcu() takes > RCU_DATA_ME().lock, so there are still code sites from which a printk > can deadlock. Only now, it is config-dependent. The RCU callbacks are (and must be) invoked without holding any RCU locks, so printk()s in RCU callbacks are perfectly permissible. It would not be -that- hard to eliminate the lock in call_rcu(), but yeccch... It would be far easier to debug RCU without the benefit printk()s than to deal with a preemptable RCU implementation that was based solely on atomic instructions!!! > >From a quick look it appears that large amounts of kernel/rcupreempt.c > are now a printk-free zone. The only printk() is in preemptable RCU's rcu_init() function, whose only purpose is to mark the dmesg appropriately, and which should not be a problem. And that would be because I do a fair amount of prototyping and debugging in user space, where one instead uses printf(). ;-) Thanx, Paul -- 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/