linux-kernel - [PATCH v2 02/45] CPU hotplug: Clarify the usage of different synchronization APIs

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Message-ID: <20130625202601.16593.54501.stgit@srivatsabhat.in.ibm.com>
Date:	Wed, 26 Jun 2013 01:56:01 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	tglx@...utronix.de, peterz@...radead.org, tj@...nel.org,
	oleg@...hat.com, paulmck@...ux.vnet.ibm.com, rusty@...tcorp.com.au,
	mingo@...nel.org, akpm@...ux-foundation.org, namhyung@...nel.org,
	walken@...gle.com, vincent.guittot@...aro.org, laijs@...fujitsu.com
Cc:	rostedt@...dmis.org, wangyun@...ux.vnet.ibm.com,
	xiaoguangrong@...ux.vnet.ibm.com, sbw@....edu, fweisbec@...il.com,
	zhong@...ux.vnet.ibm.com, nikunj@...ux.vnet.ibm.com,
	srivatsa.bhat@...ux.vnet.ibm.com, linux-pm@...r.kernel.org,
	linux-arch@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Subject: [PATCH v2 02/45] CPU hotplug: Clarify the usage of different
 synchronization APIs

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 <tglx@...utronix.de>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>
Cc: "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>
---

 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)
 {

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