[PATCH v2 47/52] trace, ring-buffer: Fix CPU hotplug callback registration

From: Srivatsa S. Bhat
Date: Fri Feb 14 2014 - 03:05:41 EST


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@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Acked-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
---

kernel/trace/ring_buffer.c | 19 +++++++++++--------
1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 294b8a2..0893233 100644
--- 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);

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