From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Srivatsa S. Bhat" Subject: Re: [RFC PATCH v3 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context Date: Tue, 11 Dec 2012 18:34:11 +0530 Message-ID: <50C72F4B.9090805@linux.vnet.ibm.com> References: <20121207173702.27305.1486.stgit@srivatsabhat.in.ibm.com> <20121207173759.27305.84316.stgit@srivatsabhat.in.ibm.com> <20121209205733.GA7038@redhat.com> <50C570F9.2020801@linux.vnet.ibm.com> <20121210181521.GA30684@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: Received: from e28smtp04.in.ibm.com ([122.248.162.4]:50712 "EHLO e28smtp04.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753139Ab2LKNFt (ORCPT ); Tue, 11 Dec 2012 08:05:49 -0500 Received: from /spool/local by e28smtp04.in.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Tue, 11 Dec 2012 18:35:26 +0530 In-Reply-To: <20121210181521.GA30684@redhat.com> Sender: linux-pm-owner@vger.kernel.org List-Id: linux-pm@vger.kernel.org To: Oleg Nesterov Cc: rostedt@goodmis.org, 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, 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 On 12/10/2012 11:45 PM, Oleg Nesterov wrote: > On 12/10, Srivatsa S. Bhat wrote: >> >> On 12/10/2012 02:27 AM, Oleg Nesterov wrote: >>> However. If this is true, then compared to preempt_disable/stop_machine >>> livelock is possible. Probably this is fine, we have the same problem with >>> get_online_cpus(). But if we can accept this fact I feel we can simmplify >>> this somehow... Can't prove, only feel ;) >> >> Not sure I follow.. > > I meant that write_lock_irqsave(&hotplug_rwlock) in take_cpu_down() > can spin "forever". > > Suppose that reader_acked() == T on every CPU, so that > get_online_cpus_atomic() always takes read_lock(&hotplug_rwlock). > > It is possible that this lock will be never released by readers, > > CPU_0 CPU_1 > > get_online_cpus_atomic() > get_online_cpus_atomic() > put_online_cpus_atomic() > > get_online_cpus_atomic() > put_online_cpus_atomic() > > get_online_cpus_atomic() > put_online_cpus_atomic() > > and so on. > Right, and we can't do anything about it :( Regards, Srivatsa S. Bhat