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>

1 files changed, 4 insertions, 4 deletions

diff --git a/mm/zswap.c b/mm/zswap.c
index e55bab9dc41f..d7337fbf6605 100644
--- 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;
 }