From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1761234Ab3BJUWg (ORCPT ); Sun, 10 Feb 2013 15:22:36 -0500 Received: from e23smtp03.au.ibm.com ([202.81.31.145]:60513 "EHLO e23smtp03.au.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1761188Ab3BJUWd (ORCPT ); Sun, 10 Feb 2013 15:22:33 -0500 Message-ID: <511800FA.1060007@linux.vnet.ibm.com> Date: Mon, 11 Feb 2013 01:50:10 +0530 From: "Srivatsa S. Bhat" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:15.0) Gecko/20120828 Thunderbird/15.0 MIME-Version: 1.0 To: Oleg Nesterov CC: linux-doc@vger.kernel.org, peterz@infradead.org, fweisbec@gmail.com, mingo@kernel.org, linux-arch@vger.kernel.org, linux@arm.linux.org.uk, 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, rjw@sisk.pl, namhyung@kernel.org, tglx@linutronix.de, linux-arm-kernel@lists.infradead.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, sbw@mit.edu, tj@kernel.org, akpm@linux-foundation.org, linuxppc-dev@lists.ozlabs.org Subject: Re: [PATCH v5 04/45] percpu_rwlock: Implement the core design of Per-CPU Reader-Writer Locks References: <20130122073210.13822.50434.stgit@srivatsabhat.in.ibm.com> <20130122073347.13822.85876.stgit@srivatsabhat.in.ibm.com> <20130208231017.GK2666@linux.vnet.ibm.com> <5117F0C0.2030605@linux.vnet.ibm.com> <20130210201312.GB6236@redhat.com> In-Reply-To: <20130210201312.GB6236@redhat.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit X-Content-Scanned: Fidelis XPS MAILER x-cbid: 13021020-6102-0000-0000-000002FAF41D Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 02/11/2013 01:43 AM, Oleg Nesterov wrote: > On 02/11, Srivatsa S. Bhat wrote: >> >> On 02/09/2013 04:40 AM, Paul E. McKenney wrote: >>>> +static void announce_writer_inactive(struct percpu_rwlock *pcpu_rwlock) >>>> +{ >>>> + unsigned int cpu; >>>> + >>>> + drop_writer_signal(pcpu_rwlock, smp_processor_id()); >>> >>> Why do we drop ourselves twice? More to the point, why is it important to >>> drop ourselves first? >>> >> >> I don't see where we are dropping ourselves twice. Note that we are no longer >> in the cpu_online_mask, so the 'for' loop below won't include us. So we need >> to manually drop ourselves. It doesn't matter whether we drop ourselves first >> or later. > > Yes, but this just reflects its usage in cpu-hotplug. cpu goes away under > _write_lock. > Ah, right. I guess the code still has remnants from the older version in which this locking scheme wasn't generic and was tied to cpu-hotplug alone.. > Perhaps _write_lock/unlock shoud use for_each_possible_cpu() instead? > Hmm, that wouldn't be too bad. > Hmm... I think this makes sense anyway. Otherwise, in theory, > percpu_write_lock(random_non_hotplug_lock) can race with cpu_up? > Yeah, makes sense. Will change it to for_each_possible_cpu(). And I had previously fixed such races with lglocks with a complicated scheme (to avoid the costly for_each_possible loop), which was finally rewritten to use for_each_possible_cpu() for the sake of simplicity.. Regards, Srivatsa S. Bhat From mboxrd@z Thu Jan 1 00:00:00 1970 From: srivatsa.bhat@linux.vnet.ibm.com (Srivatsa S. Bhat) Date: Mon, 11 Feb 2013 01:50:10 +0530 Subject: [PATCH v5 04/45] percpu_rwlock: Implement the core design of Per-CPU Reader-Writer Locks In-Reply-To: <20130210201312.GB6236@redhat.com> References: <20130122073210.13822.50434.stgit@srivatsabhat.in.ibm.com> <20130122073347.13822.85876.stgit@srivatsabhat.in.ibm.com> <20130208231017.GK2666@linux.vnet.ibm.com> <5117F0C0.2030605@linux.vnet.ibm.com> <20130210201312.GB6236@redhat.com> Message-ID: <511800FA.1060007@linux.vnet.ibm.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On 02/11/2013 01:43 AM, Oleg Nesterov wrote: > On 02/11, Srivatsa S. Bhat wrote: >> >> On 02/09/2013 04:40 AM, Paul E. McKenney wrote: >>>> +static void announce_writer_inactive(struct percpu_rwlock *pcpu_rwlock) >>>> +{ >>>> + unsigned int cpu; >>>> + >>>> + drop_writer_signal(pcpu_rwlock, smp_processor_id()); >>> >>> Why do we drop ourselves twice? More to the point, why is it important to >>> drop ourselves first? >>> >> >> I don't see where we are dropping ourselves twice. Note that we are no longer >> in the cpu_online_mask, so the 'for' loop below won't include us. So we need >> to manually drop ourselves. It doesn't matter whether we drop ourselves first >> or later. > > Yes, but this just reflects its usage in cpu-hotplug. cpu goes away under > _write_lock. > Ah, right. I guess the code still has remnants from the older version in which this locking scheme wasn't generic and was tied to cpu-hotplug alone.. > Perhaps _write_lock/unlock shoud use for_each_possible_cpu() instead? > Hmm, that wouldn't be too bad. > Hmm... I think this makes sense anyway. Otherwise, in theory, > percpu_write_lock(random_non_hotplug_lock) can race with cpu_up? > Yeah, makes sense. Will change it to for_each_possible_cpu(). And I had previously fixed such races with lglocks with a complicated scheme (to avoid the costly for_each_possible loop), which was finally rewritten to use for_each_possible_cpu() for the sake of simplicity.. Regards, Srivatsa S. Bhat