Date: Thu, 17 Nov 2016 07:54:23 -0800
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Boqun Feng <boqun.feng@gmail.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@kernel.org>,
        dipankar@in.ibm.com, akpm@linux-foundation.org,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Josh Triplett <josh@joshtriplett.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        David Howells <dhowells@redhat.com>, edumazet@google.com,
        dvhart@linux.intel.com,
        =?iso-8859-1?Q?Fr=E9d=E9ric?= Weisbecker <fweisbec@gmail.com>,
        oleg@redhat.com, bobby.prani@gmail.com, ldr709@gmail.com
Subject: Re: [PATCH RFC tip/core/rcu] SRCU rewrite
Reply-To: paulmck@linux.vnet.ibm.com
References: <20161114183636.GA28589@linux.vnet.ibm.com>
 <20161115014445.GC12110@tardis.cn.ibm.com>
 <20161115143700.GZ4127@linux.vnet.ibm.com>
 <CAJhGHyCr74zTnc10QvORDZXfd-vdc0Caa0hG250UWwMwsMkUfQ@mail.gmail.com>
 <20161117134957.GW3612@linux.vnet.ibm.com>
 <20161117143829.GA21600@linux.vnet.ibm.com>
 <20161117144544.GC5227@tardis.cn.ibm.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20161117144544.GC5227@tardis.cn.ibm.com>
User-Agent: Mutt/1.5.21 (2010-09-15)
Message-Id: <20161117155423.GA3612@linux.vnet.ibm.com>
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1994
Lines: 45

On Thu, Nov 17, 2016 at 10:45:44PM +0800, Boqun Feng wrote:
> On Thu, Nov 17, 2016 at 06:38:29AM -0800, Paul E. McKenney wrote:
> > On Thu, Nov 17, 2016 at 05:49:57AM -0800, Paul E. McKenney wrote:
> > > On Thu, Nov 17, 2016 at 08:18:51PM +0800, Lai Jiangshan wrote:
> > > > On Tue, Nov 15, 2016 at 10:37 PM, Paul E. McKenney
> > > > <paulmck@linux.vnet.ibm.com> wrote:
> > > > > On Tue, Nov 15, 2016 at 09:44:45AM +0800, Boqun Feng wrote:
> > > > 
> > > > >>
> > > > >> __srcu_read_lock() used to be called with preemption disabled. I guess
> > > > >> the reason was because we have two percpu variables to increase. So with
> > > > >> only one percpu right, could we remove the preempt_{dis,en}able() in
> > > > >> srcu_read_lock() and use this_cpu_inc() here?
> > > > >
> > > > > Quite possibly...
> > > > >
> > > > 
> > > > it will be nicer if it is removed.
> > > > 
> > > > The reason for the preemption-disabled was also because we
> > > > have to disallow any preemption between the fetching of the idx
> > > > and the increasement. so that we have at most NR_CPUS worth
> > > > of readers using the old index that haven't incremented the counters.
> > > > 
> > > > if we remove the preempt_{dis,en}able(). we must change the
> > > > "NR_CPUS" in the comment into ULONG_MAX/4. (I assume
> > > > one on-going reader needs at least need 4bytes at the stack). it is still safe.
> > > > 
> > > > but we still need to think more if we want to remove the preempt_{dis,en}able().
> > > 
> > > Good points!  Agreed, any change in the preemption needs careful thought
> > > and needs to be a separate patch.
> > 
> > And one area needing special thought is the call to __srcu_read_lock()
> > and __srcu_read_unlock() in do_exit().
> > 
> 
> So before commit 49f5903b473c5, we don't have the read of ->completed in
> preemption disable section?
> 
> And following "git blame", I found commit 7a6b55e7108b3 ;-)

Thankfully the first (49f5903b473c5) came after the second.  ;-)

							Thanx, Paul