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Message-ID: <20130623133808.19094.62963.stgit@srivatsabhat.in.ibm.com>
Date: Sun, 23 Jun 2013 19:08:12 +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>
Subject: [PATCH 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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