[v3,50/52] mm, zswap: Fix CPU hotplug callback registration

Commit Message

Srivatsa S. Bhat March 10, 2014, 8:42 p.m. UTC

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 <[e-mail]>
Cc: [e-mail]
Signed-off-by: Srivatsa S. Bhat <[e-mail]>
---

 mm/zswap.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)


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Patch

diff --git a/mm/zswap.c b/mm/zswap.c
index e55bab9..d7337fb 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;
 }