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Doc/cpu-hotplug: Specify race-free way to register CPU hotplug callbacks

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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>
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Srivatsa S. Bhat 2014-03-11 02:04:32 +05:30 • committed by Rafael J. Wysocki
parent 93ae4f978c
commit 8489d90b5f
1 changed files with 45 additions and 0 deletions
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45
Documentation/cpu-hotplug.txt
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@ -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? 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. 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 If you need to perform some action for each cpu already in the system, then
do this:
for_each_online_cpu(i) { for_each_online_cpu(i) {
foobar_cpu_callback(&foobar_cpu_notifier, CPU_UP_PREPARE, i); foobar_cpu_callback(&foobar_cpu_notifier, CPU_UP_PREPARE, i);
foobar_cpu_callback(&foobar_cpu_notifier, CPU_ONLINE, 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, Q: If i would like to develop cpu hotplug support for a new architecture,
what do i need at a minimum? what do i need at a minimum?
A: The following are what is required for CPU hotplug infrastructure to work A: The following are what is required for CPU hotplug infrastructure to work