[PATCH v3 48/52] profile: Fix CPU hotplug callback registration

["Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>]

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 profile code by using this latter form of callback registration.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Mauro Carvalho Chehab <mchehab@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 kernel/profile.c |   20 +++++++++++++++-----
 1 file changed, 15 insertions(+), 5 deletions(-)

diff --git a/kernel/profile.c b/kernel/profile.c
index 6631e1e..4c0cb95 100644
--- a/kernel/profile.c
+++ b/kernel/profile.c
@@ -591,18 +591,28 @@ out_cleanup:
 int __ref create_proc_profile(void) /* false positive from hotcpu_notifier */
 {
 	struct proc_dir_entry *entry;
+	int err = 0;
 
 	if (!prof_on)
 		return 0;
-	if (create_hash_tables())
-		return -ENOMEM;
+
+	cpu_notifier_register_begin();
+
+	if (create_hash_tables()) {
+		err = -ENOMEM;
+		goto out;
+	}
+
 	entry = proc_create("profile", S_IWUSR | S_IRUGO,
 			    NULL, &proc_profile_operations);
 	if (!entry)
-		return 0;
+		goto out;
 	proc_set_size(entry, (1 + prof_len) * sizeof(atomic_t));
-	hotcpu_notifier(profile_cpu_callback, 0);
-	return 0;
+	__hotcpu_notifier(profile_cpu_callback, 0);
+
+out:
+	cpu_notifier_register_done();
+	return err;
 }
 module_init(create_proc_profile);
 #endif /* CONFIG_PROC_FS */