From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751484AbdAPHkE (ORCPT ); Mon, 16 Jan 2017 02:40:04 -0500 Received: from relay3-d.mail.gandi.net ([217.70.183.195]:40270 "EHLO relay3-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750862AbdAPHkC (ORCPT ); Mon, 16 Jan 2017 02:40:02 -0500 X-Originating-IP: 50.39.169.170 Date: Sun, 15 Jan 2017 23:39:51 -0800 From: Josh Triplett To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, dvhart@linux.intel.com, fweisbec@gmail.com, oleg@redhat.com, bobby.prani@gmail.com Subject: Re: [PATCH tip/core/rcu 1/6] rcu: Abstract the dynticks momentary-idle operation Message-ID: <20170116073951.ipupoxcjnv42dwop@x> References: <20170114085406.GA19052@linux.vnet.ibm.com> <1484384085-19444-1-git-send-email-paulmck@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1484384085-19444-1-git-send-email-paulmck@linux.vnet.ibm.com> User-Agent: NeoMutt/20161126 (1.7.1) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Jan 14, 2017 at 12:54:40AM -0800, Paul E. McKenney wrote: > This commit is the first step towards full abstraction of all accesses to > the ->dynticks counter, implementing the previously open-coded atomic add > of two in a new rcu_dynticks_momentary_idle() function. This abstraction > will ease changes to the ->dynticks counter operation. > > Signed-off-by: Paul E. McKenney This change has an additional effect not documented in the commit message: it eliminates the smp_mb__before_atomic and smp_mb__after_atomic calls. Can you please document that in the commit message, and explain why that doesn't cause a problem? > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -281,6 +281,19 @@ static DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = { > #endif /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */ > }; > > +/* > + * Do a double-increment of the ->dynticks counter to emulate a > + * momentary idle-CPU quiescent state. > + */ > +static void rcu_dynticks_momentary_idle(void) > +{ > + struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks); > + int special = atomic_add_return(2, &rdtp->dynticks); > + > + /* It is illegal to call this from idle state. */ > + WARN_ON_ONCE(!(special & 0x1)); > +} > + > DEFINE_PER_CPU_SHARED_ALIGNED(unsigned long, rcu_qs_ctr); > EXPORT_PER_CPU_SYMBOL_GPL(rcu_qs_ctr); > > @@ -300,7 +313,6 @@ EXPORT_PER_CPU_SYMBOL_GPL(rcu_qs_ctr); > static void rcu_momentary_dyntick_idle(void) > { > struct rcu_data *rdp; > - struct rcu_dynticks *rdtp; > int resched_mask; > struct rcu_state *rsp; > > @@ -327,10 +339,7 @@ static void rcu_momentary_dyntick_idle(void) > * quiescent state, with no need for this CPU to do anything > * further. > */ > - rdtp = this_cpu_ptr(&rcu_dynticks); > - smp_mb__before_atomic(); /* Earlier stuff before QS. */ > - atomic_add(2, &rdtp->dynticks); /* QS. */ > - smp_mb__after_atomic(); /* Later stuff after QS. */ > + rcu_dynticks_momentary_idle(); > break; > } > } > -- > 2.5.2 >