From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1751369AbdAWVd6 (ORCPT <rfc822;w@1wt.eu>);
        Mon, 23 Jan 2017 16:33:58 -0500
Received: from mail-pf0-f196.google.com ([209.85.192.196]:35870 "EHLO
        mail-pf0-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1750945AbdAWVd5 (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Mon, 23 Jan 2017 16:33:57 -0500
Date: Mon, 23 Jan 2017 13:33:59 -0800
From: Lance Roy <ldr709@gmail.com>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
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, 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
Subject: Re: [PATCH] SRCU: More efficient reader counts.
Message-ID: <20170123133359.2d69756a@gmail.com>
In-Reply-To: <20170123203508.GQ28085@linux.vnet.ibm.com>
References: <1484520155-21017-1-git-send-email-paulmck@linux.vnet.ibm.com>
        <20170123201725.27392-1-ldr709@gmail.com>
        <20170123201725.27392-2-ldr709@gmail.com>
        <20170123203508.GQ28085@linux.vnet.ibm.com>
X-Mailer: Claws Mail 3.13.2 (GTK+ 2.24.30; x86_64-pc-linux-gnu)
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Mon, 23 Jan 2017 12:35:08 -0800
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:

> On Mon, Jan 23, 2017 at 12:17:25PM -0800, Lance Roy wrote:
> > SRCU uses two per-cpu counters: a nesting counter to count the number of
> > active critical sections, and a sequence counter to ensure that the nesting
> > counters don't change while they are being added together in
> > srcu_readers_active_idx_check().
> > 
> > This patch instead uses per-cpu lock and unlock counters. Because the both
> > counters only increase and srcu_readers_active_idx_check() reads the unlock
> > counter before the lock counter, this achieves the same end without having
> > to increment two different counters in srcu_read_lock(). This also saves a
> > smp_mb() in srcu_readers_active_idx_check().
> > 
> > Possible bug: There is no guarantee that the lock counter won't overflow
> > during srcu_readers_active_idx_check(), as there are no memory barriers
> > around srcu_flip() (see comment in srcu_readers_active_idx_check() for
> > details). However, this problem was already present before this patch.
> > 
> > Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> > Signed-off-by: Lance Roy <ldr709@gmail.com>  
> 
> In general, the comment update looks good, but this patch undoes my
> application of review feedback to your original patch.  Could you please
> submit the comment update as a separate patch on top of branch rcu/next
> of my -rcu tree?
> 
> git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git
I have added the review feedback to the new patch. I will send the new version.
If you prefer that the comment changes are in a separate patch I can change it,
but I thought it would be better not to have an incorrect statement in the
commit message.

> Of course, if any of the changes from the review feedback are in any
> way problematic, please do let me know.
I don't see any problems.

Thanks,
Lance