From mboxrd@z Thu Jan 1 00:00:00 1970 From: Oleg Nesterov Subject: Re: [RFC PATCH v4 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context Date: Wed, 12 Dec 2012 19:23:08 +0100 Message-ID: <20121212182308.GA26094@redhat.com> References: <20121211140314.23621.64088.stgit@srivatsabhat.in.ibm.com> <20121211140358.23621.97011.stgit@srivatsabhat.in.ibm.com> <20121212171720.GA22289@redhat.com> <20121212172431.GA23328@redhat.com> <50C8C8C9.2070605@linux.vnet.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <50C8C8C9.2070605@linux.vnet.ibm.com> Sender: linux-kernel-owner@vger.kernel.org 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 List-Id: linux-pm@vger.kernel.org On 12/12, Srivatsa S. Bhat wrote: > > On 12/12/2012 10:54 PM, Oleg Nesterov wrote: > > > 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(). > > Hmm.. > > > At the same time reader_nested_percpu() can be checked before mb(). > > I thought that since the increment and the check (reader_nested_percpu) > act on the same memory location, they will naturally be run in the given > order, without any need for barriers. Am I wrong? And this is what I meant, you do not need a barrier before reader_nested_percpu(). But you need to ensure that WRITE(reader_percpu_refcnt) and READ(writer_signal) can't be reordered, so you need mb() in between. rmb() can serialize LOADs and STOREs. Or I misunderstood? Oleg.