Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756286AbaBFLoB (ORCPT ); Thu, 6 Feb 2014 06:44:01 -0500 Received: from e28smtp05.in.ibm.com ([122.248.162.5]:46482 "EHLO e28smtp05.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755949AbaBFLoA (ORCPT ); Thu, 6 Feb 2014 06:44:00 -0500 Message-ID: <52F3743D.3030105@linux.vnet.ibm.com> Date: Thu, 06 Feb 2014 17:08:37 +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: Will Deacon CC: "paulus@samba.org" , "oleg@redhat.com" , "rusty@rustcorp.com.au" , "peterz@infradead.org" , "tglx@linutronix.de" , "akpm@linux-foundation.org" , "mingo@kernel.org" , "paulmck@linux.vnet.ibm.com" , "tj@kernel.org" , "walken@google.com" , "ego@linux.vnet.ibm.com" , "linux@arm.linux.org.uk" , "linux-kernel@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" Subject: Re: [PATCH 08/51] arm, hw-breakpoint: Fix CPU hotplug callback registration References: <20140205220251.19080.92336.stgit@srivatsabhat.in.ibm.com> <20140205220603.19080.80971.stgit@srivatsabhat.in.ibm.com> <20140206105718.GG26035@mudshark.cambridge.arm.com> <52F3713A.3040407@linux.vnet.ibm.com> <20140206113933.GJ26035@mudshark.cambridge.arm.com> In-Reply-To: <20140206113933.GJ26035@mudshark.cambridge.arm.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14020611-8256-0000-0000-00000B54505E Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 02/06/2014 05:09 PM, Will Deacon wrote: > On Thu, Feb 06, 2014 at 11:25:46AM +0000, Srivatsa S. Bhat wrote: >> Hi Will, > > Hello, > >> On 02/06/2014 04:27 PM, Will Deacon wrote: >>> On Wed, Feb 05, 2014 at 10:06:04PM +0000, Srivatsa S. Bhat wrote: >>>> Subsystems that want to register CPU hotplug callbacks, as well as perform >>>> initialization for the CPUs that are already online, often do it as shown >>>> below: >>>> >>>> get_online_cpus(); >>>> >>>> for_each_online_cpu(cpu) >>>> init_cpu(cpu); >>>> >>>> register_cpu_notifier(&foobar_cpu_notifier); >>>> >>>> put_online_cpus(); >>>> >>>> This is wrong, since it is prone to ABBA deadlocks involving the >>>> cpu_add_remove_lock and the cpu_hotplug.lock (when running concurrently >>>> with CPU hotplug operations). >>> >>> Hmm, the code in question (for this patch) runs from an arch_initcall. How >>> can you generate CPU hotplug operations at that stage? >>> >> >> You are right - in today's design of the init sequence, CPU hotplug >> operations can't be triggered at that time during boot. > > Phew, so we don't have a bug as the code stands today. Yes, that's right. > >> However, there have been proposals to boot CPUs in parallel along with the >> rest of the kernel initialization [1] (and that would need full synchronization >> with CPU hotplug even at the initcall stage). Of course this needs a lot of >> auditing and modifications to the CPU hotplug notifiers of various subsystems >> to make them robust enough to handle the parallel boot; so its not going to >> happen very soon. But I felt that it would be a good idea to ensure that we >> get the locking/synchronization right, even if the registrations happen very >> early during boot today.. you know, just to be on the safer side and also to >> make the job easier for whoever that is, who tries to implement parallel >> CPU booting again in the future ;-) >> >> [1]. http://thread.gmane.org/gmane.linux.kernel/1246209 > > Makes sense, and this seems like a good start. > >>> so it's best if you take this all via your tree. >>> >> >> Hmm.. I'm not a maintainer myself, so I'm hoping that either Oleg or Rusty >> or any of the other CPU hotplug maintainers (Thomas/Peter/Ingo) would be >> willing to take these patches through their tree. > > Well, you can have my ack for this patch: > > Acked-by: Will Deacon > Great! Thanks a lot Will! Regards, Srivatsa S. Bhat -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/