linux-kernel - [PATCH 02/51] Doc/cpu-hotplug: Specify race-free way to register CPU hotplug callbacks

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Date:	Thu, 06 Feb 2014 03:34:58 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	paulus@...ba.org, oleg@...hat.com, rusty@...tcorp.com.au,
	peterz@...radead.org, tglx@...utronix.de, akpm@...ux-foundation.org
Cc:	mingo@...nel.org, paulmck@...ux.vnet.ibm.com, tj@...nel.org,
	walken@...gle.com, ego@...ux.vnet.ibm.com, linux@....linux.org.uk,
	linux-kernel@...r.kernel.org, srivatsa.bhat@...ux.vnet.ibm.com,
	Rob Landley <rob@...dley.net>, linux-doc@...r.kernel.org,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Subject: [PATCH 02/51] Doc/cpu-hotplug: Specify race-free way to register CPU
 hotplug callbacks

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 <rob@...dley.net>
Cc: linux-doc@...r.kernel.org
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>
---

 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

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