From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757324AbaIIQQz (ORCPT ); Tue, 9 Sep 2014 12:16:55 -0400 Received: from e36.co.us.ibm.com ([32.97.110.154]:39706 "EHLO e36.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751779AbaIIQQx (ORCPT ); Tue, 9 Sep 2014 12:16:53 -0400 Date: Tue, 9 Sep 2014 09:16:48 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: Andreea-Cristina Bernat , paulus@samba.org, mingo@redhat.com, acme@kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] kernel: events: core: Replace rcu_assign_pointer() with RCU_INIT_POINTER() Message-ID: <20140909161648.GE4704@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20140822132605.GA20130@ada> <20140909094235.GD19379@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140909094235.GD19379@twins.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14090916-3532-0000-0000-0000047D1726 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Sep 09, 2014 at 11:42:35AM +0200, Peter Zijlstra wrote: > On Fri, Aug 22, 2014 at 04:26:05PM +0300, Andreea-Cristina Bernat wrote: > > The use of "rcu_assign_pointer()" is NULLing out the pointer. > > According to RCU_INIT_POINTER()'s block comment: > > "1. This use of RCU_INIT_POINTER() is NULLing out the pointer" > > it is better to use it instead of rcu_assign_pointer() because it has a > > smaller overhead. > > > > The following Coccinelle semantic patch was used: > > @@ > > @@ > > > > - rcu_assign_pointer > > + RCU_INIT_POINTER > > (..., NULL) > > > > Signed-off-by: Andreea-Cristina Bernat > > --- > > kernel/events/core.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/kernel/events/core.c b/kernel/events/core.c > > index 0d735be..4b84dd5 100644 > > --- a/kernel/events/core.c > > +++ b/kernel/events/core.c > > @@ -5744,7 +5744,7 @@ static void swevent_hlist_release(struct swevent_htable *swhash) > > if (!hlist) > > return; > > > > - rcu_assign_pointer(swhash->swevent_hlist, NULL); > > + RCU_INIT_POINTER(swhash->swevent_hlist, NULL); > > kfree_rcu(hlist, rcu_head); > > } > > Paul, why not do something like the below and do away with all this > nonsense? We used to do that, but the compilers became uncooperative, despite Stephen Hemminger's best efforts. Thanx, Paul > --- > include/linux/rcupdate.h | 8 +++++++- > 1 file changed, 7 insertions(+), 1 deletion(-) > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h > index d231aa17b1d7..38c3e629ae70 100644 > --- a/include/linux/rcupdate.h > +++ b/include/linux/rcupdate.h > @@ -623,7 +623,13 @@ static inline void rcu_preempt_sleep_check(void) > * please be careful when making changes to rcu_assign_pointer() and the > * other macros that it invokes. > */ > -#define rcu_assign_pointer(p, v) smp_store_release(&p, RCU_INITIALIZER(v)) > +#define rcu_assign_pointer(p, v) \ > +do { \ > + if (__builtin_constant_p(v) && v == NULL) \ > + p = RCU_INITIALIZER(v); \ > + else \ > + smp_store_release(&p, RCU_INITIALIZER(v)); \ > +} while (0) > > /** > * rcu_access_pointer() - fetch RCU pointer with no dereferencing