From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755161AbaHLVjo (ORCPT ); Tue, 12 Aug 2014 17:39:44 -0400 Received: from e36.co.us.ibm.com ([32.97.110.154]:59053 "EHLO e36.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751410AbaHLVjm (ORCPT ); Tue, 12 Aug 2014 17:39:42 -0400 Date: Tue, 12 Aug 2014 14:39:36 -0700 From: "Paul E. McKenney" To: Amit Shah Cc: linux-kernel@vger.kernel.org, riel@redhat.com, mingo@kernel.org, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, niv@us.ibm.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, sbw@mit.edu Subject: Re: [PATCH tip/core/rcu 1/2] rcu: Parallelize and economize NOCB kthread wakeups Message-ID: <20140812213936.GA3106@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20140808214648.GA19058@linux.vnet.ibm.com> <20140811071308.GA4184@grmbl.mre> <20140811162807.GW5821@linux.vnet.ibm.com> <20140811194126.GF4184@grmbl.mre> <20140811201102.GD5821@linux.vnet.ibm.com> <20140811201845.GG4184@grmbl.mre> <20140811203421.GE5821@linux.vnet.ibm.com> <20140812034531.GA13801@linux.vnet.ibm.com> <20140812053321.GK4184@grmbl.mre> <20140812160621.GC4752@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140812160621.GC4752@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14081221-3532-0000-0000-000003CC4EA0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Aug 12, 2014 at 09:06:21AM -0700, Paul E. McKenney wrote: > On Tue, Aug 12, 2014 at 11:03:21AM +0530, Amit Shah wrote: [ . . . ] > > I know of only virtio-console doing this (via userspace only, > > though). > > As in userspace within the guest? That would not work. The userspace > that the qemu is running in might. There is a way to extract ftrace info > from crash dumps, so one approach would be "sendkey alt-sysrq-c", then > pull the buffer from the resulting dump. For all I know, there might also > be some script that uses the qemu "x" command to get at the ftrace buffer. > > Again, I cannot reproduce this, and I have been through the code several > times over the past few days, and am not seeing it. I could start > sending you random diagnostic patches, but it would be much better if > we could get the trace data from the failure. Hearing no objections, random patch #1. The compiler could in theory cause trouble without this patch, so there is some possibility that it is a fix. Thanx, Paul ------------------------------------------------------------------------ diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index a2333d07f5d6..1dc72f523c4a 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -2043,7 +2043,7 @@ static void wake_nocb_leader(struct rcu_data *rdp, bool force) if (!ACCESS_ONCE(rdp_leader->nocb_kthread)) return; if (!ACCESS_ONCE(rdp_leader->nocb_leader_wake) || force) { - /* Prior xchg orders against prior callback enqueue. */ + /* Prior smp_mb__after_atomic() orders against prior enqueue. */ ACCESS_ONCE(rdp_leader->nocb_leader_wake) = true; wake_up(&rdp_leader->nocb_wq); } @@ -2072,6 +2072,7 @@ static void __call_rcu_nocb_enqueue(struct rcu_data *rdp, ACCESS_ONCE(*old_rhpp) = rhp; atomic_long_add(rhcount, &rdp->nocb_q_count); atomic_long_add(rhcount_lazy, &rdp->nocb_q_count_lazy); + smp_mb__after_atomic(); /* Store *old_rhpp before _wake test. */ /* If we are not being polled and there is a kthread, awaken it ... */ t = ACCESS_ONCE(rdp->nocb_kthread);