From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752425AbbJFRqi (ORCPT ); Tue, 6 Oct 2015 13:46:38 -0400 Received: from relay3-d.mail.gandi.net ([217.70.183.195]:40655 "EHLO relay3-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751460AbbJFRqh (ORCPT ); Tue, 6 Oct 2015 13:46:37 -0400 Date: Tue, 6 Oct 2015 10:46:31 -0700 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, Boqun Feng Subject: Re: [PATCH tip/core/rcu 04/13] rcu: Don't disable preemption for Tiny and Tree RCU readers Message-ID: <20151006174631.GB10272@cloud> References: <20151006161305.GA9799@linux.vnet.ibm.com> <1444148028-11551-1-git-send-email-paulmck@linux.vnet.ibm.com> <1444148028-11551-4-git-send-email-paulmck@linux.vnet.ibm.com> <20151006164445.GA9600@cloud> <20151006170101.GG3910@linux.vnet.ibm.com> <20151006171630.GC9600@cloud> <20151006174204.GL3910@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151006174204.GL3910@linux.vnet.ibm.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Oct 06, 2015 at 10:42:04AM -0700, Paul E. McKenney wrote: > On Tue, Oct 06, 2015 at 10:16:30AM -0700, Josh Triplett wrote: > > On Tue, Oct 06, 2015 at 10:01:01AM -0700, Paul E. McKenney wrote: > > > On Tue, Oct 06, 2015 at 09:44:45AM -0700, Josh Triplett wrote: > > > > On Tue, Oct 06, 2015 at 09:13:39AM -0700, Paul E. McKenney wrote: > > > > > From: Boqun Feng > > > > > > > > > > Because preempt_disable() maps to barrier() for non-debug builds, > > > > > it forces the compiler to spill and reload registers. Because Tree > > > > > RCU and Tiny RCU now only appear in CONFIG_PREEMPT=n builds, these > > > > > barrier() instances generate needless extra code for each instance of > > > > > rcu_read_lock() and rcu_read_unlock(). This extra code slows down Tree > > > > > RCU and bloats Tiny RCU. > > > > > > > > > > This commit therefore removes the preempt_disable() and preempt_enable() > > > > > from the non-preemptible implementations of __rcu_read_lock() and > > > > > __rcu_read_unlock(), respectively. However, for debug purposes, > > > > > preempt_disable() and preempt_enable() are still invoked if > > > > > CONFIG_PREEMPT_COUNT=y, because this allows detection of sleeping inside > > > > > atomic sections in non-preemptible kernels. > > > > > > > > > > This is based on an earlier patch by Paul E. McKenney, fixing > > > > > a bug encountered in kernels built with CONFIG_PREEMPT=n and > > > > > CONFIG_PREEMPT_COUNT=y. > > > > > > > > This also adds explicit barrier() calls to several internal RCU > > > > functions, but the commit message doesn't explain those at all. > > > > > > To compensate for them being removed from rcu_read_lock() and > > > rcu_read_unlock(), but yes, I will update. > > > > That much seemed clear from the comments, but that doesn't explain *why* > > those functions need barriers of their own even though rcu_read_lock() > > and rcu_read_unlock() don't. > > Ah. The reason is that Tiny RCU and Tree RCU (the !PREEMPT ones) act > by implicitly extending (and, if need be, merging) the RCU read-side > critical sections to include all the code between successive quiescent > states, for example, all the code between a pair of calls to schedule(). > > Therefore, there need to be barrier() calls in the quiescent-state > functions. Some could be argued to be implicitly present due to > translation-unit boundaries, but paranoia and all that. > > Would adding that sort of explanation help? Yes, it would.