From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S964829Ab1JFMRI (ORCPT ); Thu, 6 Oct 2011 08:17:08 -0400 Received: from mail-ww0-f44.google.com ([74.125.82.44]:62603 "EHLO mail-ww0-f44.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S935810Ab1JFMRH (ORCPT ); Thu, 6 Oct 2011 08:17:07 -0400 Date: Thu, 6 Oct 2011 14:11:28 +0200 From: Frederic Weisbecker To: "Paul E. McKenney" Cc: "Kirill A. Shutemov" , linux-kernel@vger.kernel.org, Dipankar Sarma , Thomas Gleixner , Ingo Molnar , Peter Zijlstra , Lai Jiangshan , arjan.van.de.ven@intel.com Subject: Re: linux-next-20110923: warning kernel/rcutree.c:1833 Message-ID: <20111006121118.GB23375@somewhere.redhat.com> References: <20110929123040.GB3537@somewhere> <20110929171205.GA2362@linux.vnet.ibm.com> <20110930131105.GC19053@somewhere> <20110930152946.GA2397@linux.vnet.ibm.com> <20110930192438.GA7505@linux.vnet.ibm.com> <20111002230753.GB1835@somewhere> <20111003003247.GD2539@linux.vnet.ibm.com> <20111003130335.GD1835@somewhere> <20111003163036.GC2403@linux.vnet.ibm.com> <20111006005858.GA27619@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20111006005858.GA27619@linux.vnet.ibm.com> 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 Wed, Oct 05, 2011 at 05:58:58PM -0700, Paul E. McKenney wrote: > On Mon, Oct 03, 2011 at 09:30:36AM -0700, Paul E. McKenney wrote: > > On Mon, Oct 03, 2011 at 03:03:48PM +0200, Frederic Weisbecker wrote: > > > On Sun, Oct 02, 2011 at 05:32:47PM -0700, Paul E. McKenney wrote: > > > > > > -void rcu_irq_enter(void) > > > > > > +int rcu_is_cpu_idle(void) > > > > > > { > > > > > > - rcu_exit_nohz(); > > > > > > + return (atomic_read(&__get_cpu_var(rcu_dynticks).dynticks) & 0x1) == 0; > > > > > > } > > > > > > > > > > So that's not used in this patch but it's interesting for me > > > > > to backport "rcu: Detect illegal rcu dereference in extended quiescent state". > > > > > > > > Yep, that is why it is there. > > > > > > Ok. > > > > > > > > > > > > The above should be read from a preempt disabled section though > > > > > (remember "rcu: Fix preempt-unsafe debug check of rcu extended quiescent state") > > > > > > > > Yes, and that is why the last line of the header comment reads "The > > > > caller must have at least disabled preemption." Disabling preemption > > > > is not necessary in Tiny RCU because there is no other CPU for the task > > > > to go to. (Right?) > > > > > > Right. > > > > > > > > Those functions should probably lay in a separate patch. But I don't mind > > > > > much keeping the things as is and use these APIs in my next patches though. > > > > > I'll just fix the preempt enabled thing above. > > > > > > > > Or were you saying that you wish to make calls to rcu_is_cpu_idle() > > > > that have preemption enabled? > > > > > > Yeah. That's going to be called from places like rcu_read_lock_held() > > > and things like this that don't need to disable preemption themselves. > > > > > > Would be better to disable preemption from that function. > > > > Hmmm... This might be a good use for the "drive-by" per-CPU access > > functions. > > > > No, that doesn't work. We could pick up the pointer, switch to another > > CPU, the original CPU could run a task that blocks before we start running, > > and then we could incorrectly decide that we were running in idle context, > > issuing a spurious warning. This approach would only work in environments > > that (unlike the Linux kernel) mapped all the per-CPU variables to the > > same virtual address on all CPUs. (DYNIX/ptx did this, but this leads > > to other problems, like being unable to reasonably access other CPUs' > > variables. Double mapping has other issues on some architectures.) > > > > OK, agreed. I will make this function disable preemption. > > > > > > And I can split the patch easily enough while keeping the diff the same, > > > > so you should be able to do your porting on top of the existing code. > > > > > > No I'm actually pretty fine with the current state. Whether that's defined > > > in this patch or a following one is actually not important. > > > > Fair enough! > > And here is an update that might handle an irq entry/exit miscounting > problem. Thanks to Arjan van de Ven for pointing out that my earlier > approach would in fact miscount irq entries/exits in face of things like > upcalls to user-mode helpers. I'm not sure what you mean. How could the current state miscount in user-mode?