[PATCH v3 39/52] clocksource, dummy-timer: Fix CPU hotplug callback registration

From: Srivatsa S. Bhat
Date: Mon Mar 10 2014 - 16:40:52 EST


Subsystems that want to register CPU hotplug callbacks, as well as perform
initialization for the CPUs that are already online, often do it as shown
below:

get_online_cpus();

for_each_online_cpu(cpu)
init_cpu(cpu);

register_cpu_notifier(&foobar_cpu_notifier);

put_online_cpus();

This is wrong, since it is prone to ABBA deadlocks involving the
cpu_add_remove_lock and the cpu_hotplug.lock (when running concurrently
with CPU hotplug operations).

Instead, the correct and race-free way of performing the callback
registration is:

cpu_notifier_register_begin();

for_each_online_cpu(cpu)
init_cpu(cpu);

/* Note the use of the double underscored version of the API */
__register_cpu_notifier(&foobar_cpu_notifier);

cpu_notifier_register_done();


Fix the clocksource dummy-timer code by using this latter form of callback
registration.

Cc: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
---

drivers/clocksource/dummy_timer.c | 11 ++++++++---
1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/drivers/clocksource/dummy_timer.c b/drivers/clocksource/dummy_timer.c
index b3eb582..ad35725 100644
--- a/drivers/clocksource/dummy_timer.c
+++ b/drivers/clocksource/dummy_timer.c
@@ -56,14 +56,19 @@ static struct notifier_block dummy_timer_cpu_nb = {

static int __init dummy_timer_register(void)
{
- int err = register_cpu_notifier(&dummy_timer_cpu_nb);
+ int err = 0;
+
+ cpu_notifier_register_begin();
+ err = __register_cpu_notifier(&dummy_timer_cpu_nb);
if (err)
- return err;
+ goto out;

/* We won't get a call on the boot CPU, so register immediately */
if (num_possible_cpus() > 1)
dummy_timer_setup();

- return 0;
+out:
+ cpu_notifier_register_done();
+ return err;
}
early_initcall(dummy_timer_register);

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