Re: Q: perf_event && event->owner

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Tue, 2010-11-09 at 18:42 +0100, Oleg Nesterov wrote:
> On 11/09, Peter Zijlstra wrote:
> >
> > Ah,.. quite so. So how about we explicitly destroy the list when the
> > task dies?
> 
> Yes, I think it makes sense to destroy the list and set ->owner = NULL.
> If we reset the owner, we can also avoid get_task_struct().
> 
> The only problem is perf_event_release_kernel(), it can race with the
> exiting event->owner. It can do get_task_struct() under rcu lock temporary,
> just to take the mutex and remove the entry.
> 
> > > And ptrace(), it doesn't use sys_perf_event_open() to create the event.
> >
> > Right, I guess it uses kernel based things, I guess we could not add
> > kernel based counters to the list.
> 
> Agreed, another case when event->owner should be NULL.
> 
> 
> 
> Hmm. With or without these changes. Shouldn't perf_event_release_kernel()
> remove the event from list before anything else? Otherwise, afaics a thread
> which does close(event_fd) can race with creator doing prctl(EVENTS_ENABLE),
> no?

I think you're right, how about something like this?

---
Index: linux-2.6/kernel/perf_event.c
===================================================================
--- linux-2.6.orig/kernel/perf_event.c
+++ linux-2.6/kernel/perf_event.c
@@ -2234,11 +2234,6 @@ int perf_event_release_kernel(struct per
 	raw_spin_unlock_irq(&ctx->lock);
 	mutex_unlock(&ctx->mutex);
 
-	mutex_lock(&event->owner->perf_event_mutex);
-	list_del_init(&event->owner_entry);
-	mutex_unlock(&event->owner->perf_event_mutex);
-	put_task_struct(event->owner);
-
 	free_event(event);
 
 	return 0;
@@ -2254,6 +2249,12 @@ static int perf_release(struct inode *in
 
 	file->private_data = NULL;
 
+	if (event->owner) {
+		mutex_lock(&event->owner->perf_event_mutex);
+		list_del_init(&event->owner_entry);
+		mutex_unlock(&event->owner->perf_event_mutex);
+	}
+
 	return perf_event_release_kernel(event);
 }
 
@@ -5677,7 +5678,7 @@ SYSCALL_DEFINE5(perf_event_open,
 	mutex_unlock(&ctx->mutex);
 
 	event->owner = current;
-	get_task_struct(current);
+
 	mutex_lock(&current->perf_event_mutex);
 	list_add_tail(&event->owner_entry, &current->perf_event_list);
 	mutex_unlock(&current->perf_event_mutex);
@@ -5745,12 +5746,6 @@ perf_event_create_kernel_counter(struct 
 	++ctx->generation;
 	mutex_unlock(&ctx->mutex);
 
-	event->owner = current;
-	get_task_struct(current);
-	mutex_lock(&current->perf_event_mutex);
-	list_add_tail(&event->owner_entry, &current->perf_event_list);
-	mutex_unlock(&current->perf_event_mutex);
-
 	return event;
 
 err_free:
@@ -5901,8 +5896,16 @@ static void perf_event_exit_task_context
  */
 void perf_event_exit_task(struct task_struct *child)
 {
+	struct perf_event *event, *tmp;
 	int ctxn;
 
+	mutex_lock(&child->perf_event_mutex);
+	list_for_each_entry_safe(event, tmp, &child->perf_event_list,
+				 owner_entry) {
+		list_del_init(&event->owner_entry);
+	}
+	mutex_unlock(&child->perf_event_mutex);
+
 	for_each_task_context_nr(ctxn)
 		perf_event_exit_task_context(child, ctxn);
 }