From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from e28smtp09.in.ibm.com (e28smtp09.in.ibm.com [122.248.162.9]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by ozlabs.org (Postfix) with ESMTPS id 3A14A2C0168 for ; Tue, 11 Mar 2014 07:34:44 +1100 (EST) Received: from /spool/local by e28smtp09.in.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Tue, 11 Mar 2014 02:04:41 +0530 Received: from d28relay01.in.ibm.com (d28relay01.in.ibm.com [9.184.220.58]) by d28dlp02.in.ibm.com (Postfix) with ESMTP id 1931B3940060 for ; Tue, 11 Mar 2014 02:04:39 +0530 (IST) Received: from d28av02.in.ibm.com (d28av02.in.ibm.com [9.184.220.64]) by d28relay01.in.ibm.com (8.13.8/8.13.8/NCO v10.0) with ESMTP id s2AKYZ6343188452 for ; Tue, 11 Mar 2014 02:04:35 +0530 Received: from d28av02.in.ibm.com (localhost [127.0.0.1]) by d28av02.in.ibm.com (8.14.4/8.14.4/NCO v10.0 AVout) with ESMTP id s2AKYb1h001259 for ; Tue, 11 Mar 2014 02:04:38 +0530 From: "Srivatsa S. Bhat" Subject: [PATCH v3 03/52] Doc/cpu-hotplug: Specify race-free way to register CPU hotplug callbacks To: paulus@samba.org, oleg@redhat.com, mingo@kernel.org, rjw@rjwysocki.net, rusty@rustcorp.com.au, peterz@infradead.org, tglx@linutronix.de, akpm@linux-foundation.org Date: Tue, 11 Mar 2014 02:04:32 +0530 Message-ID: <20140310203432.10746.96084.stgit@srivatsabhat.in.ibm.com> In-Reply-To: <20140310203312.10746.310.stgit@srivatsabhat.in.ibm.com> References: <20140310203312.10746.310.stgit@srivatsabhat.in.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Cc: linux-arch@vger.kernel.org, ego@linux.vnet.ibm.com, walken@google.com, linux@arm.linux.org.uk, Rob Landley , linux-pm@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linuxppc-dev@ozlabs.org, "Srivatsa S. Bhat" , tj@kernel.org, paulmck@linux.vnet.ibm.com, Ingo Molnar List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , 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: Ingo Molnar 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..a0b005d 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_notifier_register_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_notifier_register_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