From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from e23smtp07.au.ibm.com (e23smtp07.au.ibm.com [202.81.31.140]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client CN "e23smtp07.au.ibm.com", Issuer "GeoTrust SSL CA" (not verified)) by ozlabs.org (Postfix) with ESMTPS id EC5F32C0092 for ; Tue, 25 Jun 2013 04:13:30 +1000 (EST) Received: from /spool/local by e23smtp07.au.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Tue, 25 Jun 2013 04:01:59 +1000 Received: from d23relay04.au.ibm.com (d23relay04.au.ibm.com [9.190.234.120]) by d23dlp02.au.ibm.com (Postfix) with ESMTP id CAAAF2BB0052 for ; Tue, 25 Jun 2013 04:13:26 +1000 (EST) Received: from d23av04.au.ibm.com (d23av04.au.ibm.com [9.190.235.139]) by d23relay04.au.ibm.com (8.13.8/8.13.8/NCO v10.0) with ESMTP id r5OHwckf58785944 for ; Tue, 25 Jun 2013 03:58:38 +1000 Received: from d23av04.au.ibm.com (loopback [127.0.0.1]) by d23av04.au.ibm.com (8.14.4/8.13.1/NCO v10.0 AVout) with ESMTP id r5OIDOdH030284 for ; Tue, 25 Jun 2013 04:13:26 +1000 Message-ID: <51C88B76.90806@linux.vnet.ibm.com> Date: Mon, 24 Jun 2013 23:39:58 +0530 From: "Srivatsa S. Bhat" MIME-Version: 1.0 To: Tejun Heo 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 Cc: peterz@infradead.org, fweisbec@gmail.com, linux-kernel@vger.kernel.org, walken@google.com, mingo@kernel.org, linux-arch@vger.kernel.org, vincent.guittot@linaro.org, xiaoguangrong@linux.vnet.ibm.com, wangyun@linux.vnet.ibm.com, paulmck@linux.vnet.ibm.com, nikunj@linux.vnet.ibm.com, linux-pm@vger.kernel.org, rusty@rustcorp.com.au, rostedt@goodmis.org, Al Viro , namhyung@kernel.org, tglx@linutronix.de, laijs@cn.fujitsu.com, zhong@linux.vnet.ibm.com, netdev@vger.kernel.org, oleg@redhat.com, sbw@mit.edu, akpm@linux-foundation.org, linuxppc-dev@lists.ozlabs.org List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , 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