Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760116AbYHHTtw (ORCPT ); Fri, 8 Aug 2008 15:49:52 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752089AbYHHTto (ORCPT ); Fri, 8 Aug 2008 15:49:44 -0400 Received: from bombadil.infradead.org ([18.85.46.34]:41964 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751048AbYHHTto (ORCPT ); Fri, 8 Aug 2008 15:49:44 -0400 Subject: Re: [PATCH] printk: robustify printk From: Peter Zijlstra To: Andrew Morton Cc: 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, paulmck@linux.vnet.ibm.com In-Reply-To: <20080808123747.0db1c5dd.akpm@linux-foundation.org> 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> <1218217257.29098.2.camel@lappy.programming.kicks-ass.net> <1218219269.29098.5.camel@lappy.programming.kicks-ass.net> <20080808121428.646a8b3c.akpm@linux-foundation.org> <1218223269.29098.12.camel@lappy.programming.kicks-ass.net> <20080808123747.0db1c5dd.akpm@linux-foundation.org> Content-Type: text/plain Date: Fri, 08 Aug 2008 21:49:27 +0200 Message-Id: <1218224967.29098.22.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: 2624 Lines: 66 On Fri, 2008-08-08 at 12:37 -0700, Andrew Morton wrote: > On Fri, 08 Aug 2008 21:21:08 +0200 > Peter Zijlstra wrote: > > > On Fri, 2008-08-08 at 12:14 -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. > > > > > > From a quick look it appears that large amounts of kernel/rcupreempt.c > > > are now a printk-free zone. > > > > Drad, missed that bit, I did look at the calling end, but forgot the > > call_rcu() end :-/ > > > > The initial printk_tick() based implementation didn't suffer this > > problem, should we revert to that scheme? > > Dunno. Perhaps we could convert RCU_DATA_ME's spinlock_t into an > rwlock and do read_lock() in call_rcu()? Then we can should be able to > call printk from inside that read_lock(), but not inside write_lock(), > which, with suitable warning comments might be acceptable. > > afacit everything in call_rcu()'s *rdp is cpu-local and is protected by > local_irq_save(). rcu_ctrlblk.completed and rcu_flipped need some > protection, but a) rdp->lock isn't sufficient anyway and b) > read_lock protection would suffice. Maybe other CPUs can alter *rdp > while __rcu_advance_callbacks() is running. > > Anyway, that's all handwaving. My point is that making rcupreempt.c > more robust and more concurrent might be an alternative fix, and might > be beneficial in its own right. Working out the details is what we > have Pauls for ;) Hehe :-) Just in case Paul shows he's human and cannot work his way around it, I just posted a new version of the printk_tick() stuff.. -- 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/