aboutsummaryrefslogtreecommitdiffstats
path: root/Documentation/cpu-hotplug.txt
diff options
context:
space:
mode:
authorSrivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>2014-03-10 20:34:32 (GMT)
committerRafael J. Wysocki <rafael.j.wysocki@intel.com>2014-03-20 12:43:40 (GMT)
commit8489d90b5f32effa402be2a4ee7636ba0c652145 (patch)
tree1e9ba0c84932ed8b8980ca8a9acee5b47757d3ed /Documentation/cpu-hotplug.txt
parent93ae4f978ca7f26d17df915ac7afc919c1dd0353 (diff)
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@landley.net> Cc: Ingo Molnar <mingo@kernel.org> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Diffstat (limited to 'Documentation/cpu-hotplug.txt')
-rw-r--r--Documentation/cpu-hotplug.txt45
1 files changed, 45 insertions, 0 deletions
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

Privacy Policy