From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754722Ab2LLRYb (ORCPT ); Wed, 12 Dec 2012 12:24:31 -0500 Received: from mx1.redhat.com ([209.132.183.28]:64182 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754570Ab2LLRY3 (ORCPT ); Wed, 12 Dec 2012 12:24:29 -0500 Date: Wed, 12 Dec 2012 18:24:31 +0100 From: Oleg Nesterov To: "Srivatsa S. Bhat" Cc: tglx@linutronix.de, peterz@infradead.org, paulmck@linux.vnet.ibm.com, rusty@rustcorp.com.au, mingo@kernel.org, akpm@linux-foundation.org, namhyung@kernel.org, vincent.guittot@linaro.org, tj@kernel.org, sbw@mit.edu, amit.kucheria@linaro.org, rostedt@goodmis.org, rjw@sisk.pl, wangyun@linux.vnet.ibm.com, xiaoguangrong@linux.vnet.ibm.com, nikunj@linux.vnet.ibm.com, linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH v4 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context Message-ID: <20121212172431.GA23328@redhat.com> References: <20121211140314.23621.64088.stgit@srivatsabhat.in.ibm.com> <20121211140358.23621.97011.stgit@srivatsabhat.in.ibm.com> <20121212171720.GA22289@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20121212171720.GA22289@redhat.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/12, Oleg Nesterov wrote: > > On 12/11, Srivatsa S. Bhat wrote: > > > > IOW, the hotplug readers just increment/decrement their per-cpu refcounts > > when no writer is active. > > plus cli/sti ;) and increment/decrement are atomic. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ OOPS, sorry I was going to say "adds mb()". And when I look at get_online_cpus_atomic() again it uses rmb(). This doesn't look correct, we need the full barrier between this_cpu_inc() and writer_active(). At the same time reader_nested_percpu() can be checked before mb(). Oleg.