mm, zswap: Fix CPU hotplug callback registration

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 zswap code by using this latter form of callback registration.

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>

diff --git a/mm/zswap.c b/mm/zswap.c
index e55bab9dc41f81ab1b6384710e918f3839e0c936..d7337fbf66053219ab0e72e5e9b2c0b7e17d2bb7 100644 (file)
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -387,18 +387,18 @@ static int zswap_cpu_init(void)
 {
        unsigned long cpu;
 
-       get_online_cpus();
+       cpu_notifier_register_begin();
        for_each_online_cpu(cpu)
                if (__zswap_cpu_notifier(CPU_UP_PREPARE, cpu) != NOTIFY_OK)
                        goto cleanup;
-       register_cpu_notifier(&zswap_cpu_notifier_block);
-       put_online_cpus();
+       __register_cpu_notifier(&zswap_cpu_notifier_block);
+       cpu_notifier_register_done();
        return 0;
 
 cleanup:
        for_each_online_cpu(cpu)
                __zswap_cpu_notifier(CPU_UP_CANCELED, cpu);
-       put_online_cpus();
+       cpu_notifier_register_done();
        return -ENOMEM;
 }