From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756829Ab1JCQay (ORCPT ); Mon, 3 Oct 2011 12:30:54 -0400 Received: from e7.ny.us.ibm.com ([32.97.182.137]:41397 "EHLO e7.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753351Ab1JCQat (ORCPT ); Mon, 3 Oct 2011 12:30:49 -0400 Date: Mon, 3 Oct 2011 09:30:36 -0700 From: "Paul E. McKenney" To: Frederic Weisbecker Cc: "Kirill A. Shutemov" , linux-kernel@vger.kernel.org, Dipankar Sarma , Thomas Gleixner , Ingo Molnar , Peter Zijlstra , Lai Jiangshan Subject: Re: linux-next-20110923: warning kernel/rcutree.c:1833 Message-ID: <20111003163036.GC2403@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20110928234633.GA3537@somewhere> <20110929005545.GT2383@linux.vnet.ibm.com> <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20111003130335.GD1835@somewhere> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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! Thanx, Paul