From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753500Ab3FXSNh (ORCPT ); Mon, 24 Jun 2013 14:13:37 -0400 Received: from e23smtp03.au.ibm.com ([202.81.31.145]:60334 "EHLO e23smtp03.au.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753008Ab3FXSNd (ORCPT ); Mon, 24 Jun 2013 14:13:33 -0400 Message-ID: <51C88B76.90806@linux.vnet.ibm.com> Date: Mon, 24 Jun 2013 23:39:58 +0530 From: "Srivatsa S. Bhat" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:15.0) Gecko/20120828 Thunderbird/15.0 MIME-Version: 1.0 To: Tejun Heo CC: tglx@linutronix.de, peterz@infradead.org, oleg@redhat.com, paulmck@linux.vnet.ibm.com, rusty@rustcorp.com.au, mingo@kernel.org, akpm@linux-foundation.org, namhyung@kernel.org, walken@google.com, vincent.guittot@linaro.org, laijs@cn.fujitsu.com, rostedt@goodmis.org, wangyun@linux.vnet.ibm.com, xiaoguangrong@linux.vnet.ibm.com, sbw@mit.edu, fweisbec@gmail.com, zhong@linux.vnet.ibm.com, nikunj@linux.vnet.ibm.com, linux-pm@vger.kernel.org, linux-arch@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Al Viro Subject: Re: [PATCH 22/45] percpu_counter: Use get/put_online_cpus_atomic() to prevent CPU offline References: <20130623133642.19094.16038.stgit@srivatsabhat.in.ibm.com> <20130623134256.19094.99231.stgit@srivatsabhat.in.ibm.com> <20130624175535.GA1918@mtj.dyndns.org> <20130624180636.GB1918@mtj.dyndns.org> In-Reply-To: <20130624180636.GB1918@mtj.dyndns.org> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit X-Content-Scanned: Fidelis XPS MAILER x-cbid: 13062418-6102-0000-0000-000003C00F75 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 06/24/2013 11:36 PM, Tejun Heo wrote: > On Mon, Jun 24, 2013 at 10:55:35AM -0700, Tejun Heo wrote: >>> @@ -105,6 +106,7 @@ s64 __percpu_counter_sum(struct percpu_counter *fbc) >>> ret += *pcount; >>> } >>> raw_spin_unlock(&fbc->lock); >>> + put_online_cpus_atomic(); >> >> I don't think this is necessary. CPU on/offlining is explicitly >> handled via the hotplug callback which synchronizes through fbc->lock. >> __percpu_counter_sum() racing with actual on/offlining doesn't affect >> correctness and adding superflous get_online_cpus_atomic() around it >> can be misleading. > > Ah, okay, so you added a debug feature which triggers warning if > online mask is accessed without synchronization. Exactly! > Yeah, that makes > sense and while the above is not strictly necessary, it probably is > better to just add it rather than suppressing the warning in a > different way. Yeah, I was beginning to scratch my head as to how to suppress the warning after I read your explanation as to why the calls to get/put_online_cpus_atomic() would be superfluous in this case... But as you said, simply invoking those functions is much simpler ;-) > Can you please at least add a comment explaining that? > Sure, will do. Thanks a lot Tejun! Regards, Srivatsa S. Bhat