trace, ring-buffer: 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 tracing ring-buffer code by using this latter form of callback
registration.

Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Acked-by: Steven Rostedt <rostedt@goodmis.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/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index fc4da2d97f9b6e280b2e29a525e478958cb495d4..c634868c2921ca39837c3a36773c36fa4a3273d5 100644 (file)
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -1301,7 +1301,7 @@ struct ring_buffer *__ring_buffer_alloc(unsigned long size, unsigned flags,
         * In that off case, we need to allocate for all possible cpus.
         */
 #ifdef CONFIG_HOTPLUG_CPU
-       get_online_cpus();
+       cpu_notifier_register_begin();
        cpumask_copy(buffer->cpumask, cpu_online_mask);
 #else
        cpumask_copy(buffer->cpumask, cpu_possible_mask);
@@ -1324,10 +1324,10 @@ struct ring_buffer *__ring_buffer_alloc(unsigned long size, unsigned flags,
 #ifdef CONFIG_HOTPLUG_CPU
        buffer->cpu_notify.notifier_call = rb_cpu_notify;
        buffer->cpu_notify.priority = 0;
-       register_cpu_notifier(&buffer->cpu_notify);
+       __register_cpu_notifier(&buffer->cpu_notify);
+       cpu_notifier_register_done();
 #endif
 
-       put_online_cpus();
        mutex_init(&buffer->mutex);
 
        return buffer;
@@ -1341,7 +1341,9 @@ struct ring_buffer *__ring_buffer_alloc(unsigned long size, unsigned flags,
 
  fail_free_cpumask:
        free_cpumask_var(buffer->cpumask);
-       put_online_cpus();
+#ifdef CONFIG_HOTPLUG_CPU
+       cpu_notifier_register_done();
+#endif
 
  fail_free_buffer:
        kfree(buffer);
@@ -1358,16 +1360,17 @@ ring_buffer_free(struct ring_buffer *buffer)
 {
        int cpu;
 
-       get_online_cpus();
-
 #ifdef CONFIG_HOTPLUG_CPU
-       unregister_cpu_notifier(&buffer->cpu_notify);
+       cpu_notifier_register_begin();
+       __unregister_cpu_notifier(&buffer->cpu_notify);
 #endif
 
        for_each_buffer_cpu(buffer, cpu)
                rb_free_cpu_buffer(buffer->buffers[cpu]);
 
-       put_online_cpus();
+#ifdef CONFIG_HOTPLUG_CPU
+       cpu_notifier_register_done();
+#endif
 
        kfree(buffer->buffers);
        free_cpumask_var(buffer->cpumask);