From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933227AbcFOWlq (ORCPT ); Wed, 15 Jun 2016 18:41:46 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:15650 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1753136AbcFOWlp (ORCPT ); Wed, 15 Jun 2016 18:41:45 -0400 X-IBM-Helo: d03dlp01.boulder.ibm.com X-IBM-MailFrom: paulmck@linux.vnet.ibm.com X-IBM-RcptTo: linux-kernel@vger.kernel.org Date: Wed, 15 Jun 2016 15:41:39 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, 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 07/12] rcu: No ordering for rcu_assign_pointer() of NULL Reply-To: paulmck@linux.vnet.ibm.com References: <20160615214550.GA4931@linux.vnet.ibm.com> <1466027173-5501-7-git-send-email-paulmck@linux.vnet.ibm.com> <20160615220339.GZ30921@twins.programming.kicks-ass.net> <20160615221258.GE30927@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160615221258.GE30927@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: 16061522-0008-0000-0000-000004D1BFD0 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 16061522-0009-0000-0000-0000386D1D09 Message-Id: <20160615224139.GA3923@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2016-06-15_13:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=0 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1604210000 definitions=main-1606150239 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jun 16, 2016 at 12:12:58AM +0200, Peter Zijlstra wrote: > On Thu, Jun 16, 2016 at 12:03:39AM +0200, Peter Zijlstra wrote: > > On Wed, Jun 15, 2016 at 02:46:08PM -0700, Paul E. McKenney wrote: > > > This commit does a compile-time check for rcu_assign_pointer() of NULL, > > > and uses WRITE_ONCE() rather than smp_store_release() in that case. > > > > > > Reported-by: Christoph Hellwig > > > Signed-off-by: Paul E. McKenney > > > --- > > > include/linux/rcupdate.h | 11 ++++++++++- > > > 1 file changed, 10 insertions(+), 1 deletion(-) > > > > > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h > > > index c61b6b9506e7..9be61e47badc 100644 > > > --- a/include/linux/rcupdate.h > > > +++ b/include/linux/rcupdate.h > > > @@ -650,7 +650,16 @@ 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) \ > > > +({ \ > > > + uintptr_t _r_a_p__v = (uintptr_t)(v); \ > > > + \ > > > + if (__builtin_constant_p(v) && (_r_a_p__v) == (uintptr_t)NULL) \ > > > + WRITE_ONCE((p), (typeof(p))(_r_a_p__v)); \ > > > + else \ > > > + smp_store_release(&p, RCU_INITIALIZER((typeof(p))_r_a_p__v)); \ > > > + _r_a_p__v; \ > > > +}) > > > > Can we pretty please right align the '\'s ? If you insist... ;-) Done. > > Also, didn't we used to do this and then reverted it again for some > > obscure reason? > > lkml.kernel.org/r/20140909094235.GD19379@twins.programming.kicks-ass.net There was indeed a compiler bug long ago that could generate spurious warnings: https://groups.google.com/forum/#!topic/linux.kernel/y2FIhJ-WVJc > What changed since then? And can we now pretty please get rid of that > RCU_INIT_POINTER() nonsense? Five years has passed, the structure of rcu_assign_pointer() has completely changed, and someone asked for the old behavior. Seemed worth a try, given the very visible nature of the gcc complaint. No complaints thus far, but then again there probably aren't that many people running -rcu. That said, I am encouraged by the lack of reports from the 0day test robot. If this goes in and there aren't any problems for some time, then I agree that shrinking the RCU API would be worthwhile. My idea of "some time" is about a year, given that it would be a real pain to push a bunch of changes throughout the kernel only to have to revert them if the old compiler bug managed to crop up again. :-/ Thanx, Paul