Re: [tip:perf/core] perf: Fix broken union in ' struct perf_event_mmap_page'

On Wed, Sep 18, 2013 at 04:07:52PM -0400, Vince Weaver wrote:
> It would be nice if there was some way to detect this change; I liked the 
> idea of a "cap_usr_fixed" bit.

How about we start using the version field for this? Arguably we should
have incremented that value every time we changed the thing but we might
as well start now.

---

diff --git a/kernel/events/core.c b/kernel/events/core.c
index dd236b6..c44d55c 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3660,6 +3660,25 @@ static void calc_timer_values(struct perf_event *event,
 	*running = ctx_time - event->tstamp_running;
 }
 
+static void perf_event_init_userpage(struct perf_event *event)
+{
+	struct perf_event_mmap_page *userpg;
+	struct ring_buffer *rb;
+
+	rcu_read_lock();
+	rb = rcu_dereference(event->rb);
+	if (!rb)
+		goto unlock;
+
+	userpg = rb->user_page;
+
+	userpg->version = 1;
+	userpg->compat_version = 0;
+
+unlock:
+	rcu_read_unlock();
+}
+
 void __weak arch_perf_update_userpage(struct perf_event_mmap_page *userpg, u64 now)
 {
 }
@@ -4044,6 +4063,7 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)
 	ring_buffer_attach(event, rb);
 	rcu_assign_pointer(event->rb, rb);
 
+	perf_event_init_userpage(event);
 	perf_event_update_userpage(event);
 
 unlock: