From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1423852Ab2LGSbt (ORCPT ); Fri, 7 Dec 2012 13:31:49 -0500 Received: from mail-pa0-f46.google.com ([209.85.220.46]:39686 "EHLO mail-pa0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1423749Ab2LGSbr (ORCPT ); Fri, 7 Dec 2012 13:31:47 -0500 Date: Fri, 7 Dec 2012 10:31:41 -0800 From: Tejun Heo 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, oleg@redhat.com, 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 v3 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context Message-ID: <20121207183141.GC2821@htj.dyndns.org> References: <20121207173702.27305.1486.stgit@srivatsabhat.in.ibm.com> <20121207173759.27305.84316.stgit@srivatsabhat.in.ibm.com> <20121207175724.GA2821@htj.dyndns.org> <50C23441.7020309@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <50C23441.7020309@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello, Srivatsa. On Fri, Dec 07, 2012 at 11:54:01PM +0530, Srivatsa S. Bhat wrote: > > lg_lock doesn't do local nesting and I'm not sure how big a deal that > > is as I don't know how many should be converted. But if nesting is an > > absolute necessity, it would be much better to implement generic > > rwlock variant (say, lg_rwlock) rather than implementing unusual > > cpuhotplug-specific percpu synchronization construct. > > To be honest, at a certain point in time while designing this, I did > realize that this was getting kinda overly complicated ;-) ... but I > wanted to see how this would actually work out when finished and get > some feedback on the same, hence I posted it out. But this also proves > that we _can_ actually compete with the flexibility of preempt_disable() > and still be safe with respect to locking, if we really want to ;-) I got confused by comparison to preempt_disable() but you're right that percpu rwlock shouldn't be able to introduce locking dependency which doesn't exist with non-percpu rwlock. ie. write locking should be atomic w.r.t. to all readers. At the simplest, this can be implemented by writer backing out all the way if try-locking any CPU fails and retrying the whole thing. That should be correct but has the potential of starving the writer. What we need here is a generic percpu-rwlock. I don't know which exact implementation strategy we should choose. Maybe your switching to global rwlock is the right solution. But, at any rate, I think it would be best to implement proper percpu-rwlock and then apply it to CPU hotplug. It's actually gonna be pretty fitting as get_online_cpus() is being converted to percpu-rwsem. IIUC, Oleg has been working on this for a while now. Oleg, what do you think? Thanks. -- tejun