Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756056AbaBEW0K (ORCPT ); Wed, 5 Feb 2014 17:26:10 -0500 Received: from e23smtp07.au.ibm.com ([202.81.31.140]:43875 "EHLO e23smtp07.au.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753957AbaBEWKW (ORCPT ); Wed, 5 Feb 2014 17:10:22 -0500 From: "Srivatsa S. Bhat" Subject: [PATCH 02/51] Doc/cpu-hotplug: Specify race-free way to register CPU hotplug callbacks To: paulus@samba.org, oleg@redhat.com, rusty@rustcorp.com.au, peterz@infradead.org, tglx@linutronix.de, akpm@linux-foundation.org Cc: 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, srivatsa.bhat@linux.vnet.ibm.com, Rob Landley , linux-doc@vger.kernel.org, "Srivatsa S. Bhat" Date: Thu, 06 Feb 2014 03:34:58 +0530 Message-ID: <20140205220457.19080.30489.stgit@srivatsabhat.in.ibm.com> In-Reply-To: <20140205220251.19080.92336.stgit@srivatsabhat.in.ibm.com> References: <20140205220251.19080.92336.stgit@srivatsabhat.in.ibm.com> User-Agent: StGIT/0.14.3 MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14020522-0260-0000-0000-00000450B659 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Recommend the usage of the new CPU hotplug callback registration APIs (__register_cpu_notifier() etc), when subsystems need to also perform initialization for already online CPUs. Provide examples of correct and race-free ways of achieving this, and point out the kinds of code that are error-prone. Cc: Rob Landley Cc: linux-doc@vger.kernel.org Signed-off-by: Srivatsa S. Bhat --- Documentation/cpu-hotplug.txt | 45 +++++++++++++++++++++++++++++++++++++++++ 1 file changed, 45 insertions(+) diff --git a/Documentation/cpu-hotplug.txt b/Documentation/cpu-hotplug.txt index be675d2..42831b7 100644 --- a/Documentation/cpu-hotplug.txt +++ b/Documentation/cpu-hotplug.txt @@ -312,12 +312,57 @@ things will happen if a notifier in path sent a BAD notify code. Q: I don't see my action being called for all CPUs already up and running? A: Yes, CPU notifiers are called only when new CPUs are on-lined or offlined. If you need to perform some action for each cpu already in the system, then + do this: for_each_online_cpu(i) { foobar_cpu_callback(&foobar_cpu_notifier, CPU_UP_PREPARE, i); foobar_cpu_callback(&foobar_cpu_notifier, CPU_ONLINE, i); } + However, if you want to register a hotplug callback, as well as perform + some initialization for CPUs that are already online, then do this: + + Version 1: (Correct) + --------- + + cpu_maps_update_begin(); + + for_each_online_cpu(i) { + foobar_cpu_callback(&foobar_cpu_notifier, + CPU_UP_PREPARE, i); + foobar_cpu_callback(&foobar_cpu_notifier, + CPU_ONLINE, i); + } + + /* Note the use of the double underscored version of the API */ + __register_cpu_notifier(&foobar_cpu_notifier); + + cpu_maps_update_done(); + + Note that the following code is *NOT* the right way to achieve this, + because it is prone to an ABBA deadlock between the cpu_add_remove_lock + and the cpu_hotplug.lock. + + Version 2: (Wrong!) + --------- + + get_online_cpus(); + + for_each_online_cpu(i) { + foobar_cpu_callback(&foobar_cpu_notifier, + CPU_UP_PREPARE, i); + foobar_cpu_callback(&foobar_cpu_notifier, + CPU_ONLINE, i); + } + + register_cpu_notifier(&foobar_cpu_notifier); + + put_online_cpus(); + + So always use the first version shown above when you want to register + callbacks as well as initialize the already online CPUs. + + Q: If i would like to develop cpu hotplug support for a new architecture, what do i need at a minimum? A: The following are what is required for CPU hotplug infrastructure to work -- 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/