From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Srivatsa S. Bhat" Subject: [PATCH v2 02/45] CPU hotplug: Clarify the usage of different synchronization APIs Date: Wed, 26 Jun 2013 01:56:01 +0530 Message-ID: <20130625202601.16593.54501.stgit@srivatsabhat.in.ibm.com> References: <20130625202452.16593.22810.stgit@srivatsabhat.in.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20130625202452.16593.22810.stgit@srivatsabhat.in.ibm.com> Sender: linux-kernel-owner@vger.kernel.org To: peterz@infradead.org, tj@kernel.org, oleg@redhat.com, paulmck@linux.vnet.ibm.com, rusty@rustcorp.com.au, mingo@kernel.org, namhyung@kernel.org, walken@google.com, vincent.guittot@linaro.org, laijs@cn.fujitsu.com Cc: rostedt@goodmis.org, wangyun@linux.vnet.ibm.com, xiaoguangrong@linux.vnet.ibm.com, sbw@mit.edu, fweisbec@gmail.com, zhong@linux.vnet.ibm.com, nikunj@linux.vnet.ibm.com, srivatsa.bhat@linux.vnet.ibm.com, linux-pm@vger.kernel.org, linux-arch@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Thomas Gleixner , Andrew Morton , Yasuaki Ishimatsu , "Rafael J. Wysocki" "Srivatsa S. Bhat" List-Id: linux-pm@vger.kernel.org We have quite a few APIs now which help synchronize with CPU hotplug. Among them, get/put_online_cpus() is the oldest and the most well-known, so no problems there. By extension, its easy to comprehend the new set : get/put_online_cpus_atomic(). But there is yet another set, which might appear tempting to use: cpu_hotplug_disable()/cpu_hotplug_enable(). Add comments to clarify that this latter set is NOT for general use and must be used only in specific cases where the requirement is really to _disable_ hotplug and not just to synchronize with it. Cc: Thomas Gleixner Cc: Andrew Morton Cc: Yasuaki Ishimatsu Cc: "Rafael J. Wysocki" Signed-off-by: Srivatsa S. Bhat --- kernel/cpu.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/kernel/cpu.c b/kernel/cpu.c index 2d03398..860f51a 100644 --- a/kernel/cpu.c +++ b/kernel/cpu.c @@ -139,6 +139,13 @@ static void cpu_hotplug_done(void) * the 'cpu_hotplug_disabled' flag. The same lock is also acquired by the * hotplug path before performing hotplug operations. So acquiring that lock * guarantees mutual exclusion from any currently running hotplug operations. + * + * Note: In most cases, this is *NOT* the function you need. If you simply + * want to avoid racing with CPU hotplug operations, use get/put_online_cpus() + * or get/put_online_cpus_atomic(), depending on the situation. + * + * This set of functions is reserved for cases where you really wish to + * _disable_ CPU hotplug and not just synchronize with it. */ void cpu_hotplug_disable(void) {