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 zsmalloc code by using this latter form of callback registration.

Cc: Minchan Kim <minchan@kernel.org>
Cc: Nitin Gupta <redacted>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: linux-mm@kvack.org
Signed-off-by: Srivatsa S. Bhat <redacted>
---

 mm/zsmalloc.c |   17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index c03ca5e..36b4591 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c

@@ -814,21 +814,32 @@ static void zs_exit(void)
 {
 	int cpu;
 
+	cpu_notifier_register_begin();
+
 	for_each_online_cpu(cpu)
 		zs_cpu_notifier(NULL, CPU_DEAD, (void *)(long)cpu);
-	unregister_cpu_notifier(&zs_cpu_nb);
+	__unregister_cpu_notifier(&zs_cpu_nb);
+
+	cpu_notifier_register_done();
 }
 
 static int zs_init(void)
 {
 	int cpu, ret;
 
-	register_cpu_notifier(&zs_cpu_nb);
+	cpu_notifier_register_begin();
+
+	__register_cpu_notifier(&zs_cpu_nb);
 	for_each_online_cpu(cpu) {
 		ret = zs_cpu_notifier(NULL, CPU_UP_PREPARE, (void *)(long)cpu);
-		if (notifier_to_errno(ret))
+		if (notifier_to_errno(ret)) {
+			cpu_notifier_register_done();
 			goto fail;
+		}
 	}
+
+	cpu_notifier_register_done();
+
 	return 0;
 fail:
 	zs_exit();