On Tue, Mar 23, 2021 at 11:41AM +0100, Marco Elver wrote:
> On Tue, 23 Mar 2021 at 11:32, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
[...]
> > > + if (parent_event) {
> > > /*
> > > + * Remove event from parent, to avoid race where the
> > > + * parent concurrently iterates through its children to
> > > + * enable, disable, or otherwise modify an event.
> > > */
> > > + mutex_lock(&parent_event->child_mutex);
> > > + list_del_init(&event->child_list);
> > > + mutex_unlock(&parent_event->child_mutex);
> > > }
> >
> > ^^^ this, right?
> >
> > But that's something perf_event_exit_event() alread does. So then you're
> > worried about the order of things.
>
> Correct. We somehow need to prohibit the parent from doing an
> event_function_call() while we potentially deactivate the context with
> perf_remove_from_context().
>
> > > +
> > > + perf_remove_from_context(event, !!event->parent * DETACH_GROUP);
> > > + perf_event_exit_event(event, ctx, current, true);
> > > }
> >
> > perf_event_release_kernel() first does perf_remove_from_context() and
> > then clears the child_list, and that makes sense because if we're there,
> > there's no external access anymore, the filedesc is gone and nobody will
> > be iterating child_list anymore.
> >
> > perf_event_exit_task_context() and perf_event_exit_event() OTOH seem to
> > rely on ctx->task == TOMBSTONE to sabotage event_function_call() such
> > that if anybody is iterating the child_list, it'll NOP out.
> >
> > But here we don't have neither, and thus need to worry about the order
> > vs child_list iteration.
> >
> > I suppose we should stick sync_child_event() in there as well.
> >
> > And at that point there's very little value in still using
> > perf_event_exit_event()... let me see if there's something to be done
> > about that.
>
> I don't mind dropping use of perf_event_exit_event() and open coding
> all of this. That would also avoid modifying perf_event_exit_event().
>
> But I leave it to you what you think is nicest.
I played a bit more with it, and the below would be the version without
using perf_event_exit_event(). Perhaps it isn't too bad.
Thanks,
-- Marco
------ >8 ------
diff --git a/kernel/events/core.c b/kernel/events/core.c
index aa47e111435e..288b61820dab 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -2165,8 +2165,9 @@ static void perf_group_detach(struct perf_event *event)
* If this is a sibling, remove it from its group.
*/
if (leader != event) {
+ leader->nr_siblings--;
list_del_init(&event->sibling_list);
- event->group_leader->nr_siblings--;
+ event->group_leader = event;
goto out;
}
@@ -2180,8 +2181,9 @@ static void perf_group_detach(struct perf_event *event)
if (sibling->event_caps & PERF_EV_CAP_SIBLING)
perf_remove_sibling_event(sibling);
- sibling->group_leader = sibling;
+ leader->nr_siblings--;
list_del_init(&sibling->sibling_list);
+ sibling->group_leader = sibling;
/* Inherit group flags from the previous leader */
sibling->group_caps = event->group_caps;
@@ -2358,10 +2360,19 @@ __perf_remove_from_context(struct perf_event *event,
static void perf_remove_from_context(struct perf_event *event, unsigned long flags)
{
struct perf_event_context *ctx = event->ctx;
+ bool remove;
lockdep_assert_held(&ctx->mutex);
- event_function_call(event, __perf_remove_from_context, (void *)flags);
+ /*
+ * There is concurrency vs remove_on_exec().
+ */
+ raw_spin_lock_irq(&ctx->lock);
+ remove = (event->attach_state & PERF_ATTACH_CONTEXT);
+ raw_spin_unlock_irq(&ctx->lock);
+
+ if (remove)
+ event_function_call(event, __perf_remove_from_context, (void *)flags);
/*
* The above event_function_call() can NO-OP when it hits
@@ -4196,43 +4207,86 @@ static void perf_event_enable_on_exec(int ctxn)
}
static void perf_remove_from_owner(struct perf_event *event);
-static void perf_event_exit_event(struct perf_event *child_event,
- struct perf_event_context *child_ctx,
- struct task_struct *child);
+static void sync_child_event(struct perf_event *child_event,
+ struct task_struct *child);
+static void free_event(struct perf_event *event);
/*
* Removes all events from the current task that have been marked
* remove-on-exec, and feeds their values back to parent events.
*/
-static void perf_event_remove_on_exec(void)
+static void perf_event_remove_on_exec(int ctxn)
{
- int ctxn;
+ struct perf_event_context *ctx, *clone_ctx = NULL;
+ struct perf_event *event, *next;
+ LIST_HEAD(free_list);
+ unsigned long flags;
+ bool modified = false;
- for_each_task_context_nr(ctxn) {
- struct perf_event_context *ctx;
- struct perf_event *event, *next;
+ ctx = perf_pin_task_context(current, ctxn);
+ if (!ctx)
+ return;
- ctx = perf_pin_task_context(current, ctxn);
- if (!ctx)
+ mutex_lock(&ctx->mutex);
+
+ if (WARN_ON_ONCE(ctx->task != current))
+ goto unlock;
+
+ list_for_each_entry_safe(event, next, &ctx->event_list, event_entry) {
+ struct perf_event *parent_event = event->parent;
+
+ if (!event->attr.remove_on_exec)
continue;
- mutex_lock(&ctx->mutex);
- list_for_each_entry_safe(event, next, &ctx->event_list, event_entry) {
- if (!event->attr.remove_on_exec)
- continue;
+ if (!is_kernel_event(event))
+ perf_remove_from_owner(event);
+
+ modified = true;
- if (!is_kernel_event(event))
- perf_remove_from_owner(event);
- perf_remove_from_context(event, DETACH_GROUP);
+ if (parent_event) {
/*
- * Remove the event and feed back its values to the
- * parent event.
+ * Remove event from parent *before* modifying contexts,
+ * to avoid race where the parent concurrently iterates
+ * through its children to enable, disable, or otherwise
+ * modify an event.
*/
- perf_event_exit_event(event, ctx, current);
+
+ sync_child_event(event, current);
+
+ WARN_ON_ONCE(parent_event->ctx->parent_ctx);
+ mutex_lock(&parent_event->child_mutex);
+ list_del_init(&event->child_list);
+ mutex_unlock(&parent_event->child_mutex);
+
+ perf_event_wakeup(parent_event);
+ put_event(parent_event);
}
- mutex_unlock(&ctx->mutex);
- put_ctx(ctx);
+
+ perf_remove_from_context(event, !!event->parent * DETACH_GROUP);
+
+ raw_spin_lock_irq(&ctx->lock);
+ WARN_ON_ONCE(ctx->is_active);
+ perf_event_set_state(event, PERF_EVENT_STATE_EXIT); /* is_event_hup() */
+ raw_spin_unlock_irq(&ctx->lock);
+
+ if (parent_event)
+ free_event(event);
+ else
+ perf_event_wakeup(event);
}
+
+ raw_spin_lock_irqsave(&ctx->lock, flags);
+ if (modified)
+ clone_ctx = unclone_ctx(ctx);
+ --ctx->pin_count;
+ raw_spin_unlock_irqrestore(&ctx->lock, flags);
+
+unlock:
+ mutex_unlock(&ctx->mutex);
+
+ put_ctx(ctx);
+ if (clone_ctx)
+ put_ctx(clone_ctx);
}
struct perf_read_data {
@@ -7581,20 +7635,18 @@ void perf_event_exec(void)
struct perf_event_context *ctx;
int ctxn;
- rcu_read_lock();
for_each_task_context_nr(ctxn) {
- ctx = current->perf_event_ctxp[ctxn];
- if (!ctx)
- continue;
-
perf_event_enable_on_exec(ctxn);
+ perf_event_remove_on_exec(ctxn);
- perf_iterate_ctx(ctx, perf_event_addr_filters_exec, NULL,
- true);
+ rcu_read_lock();
+ ctx = rcu_dereference(current->perf_event_ctxp[ctxn]);
+ if (ctx) {
+ perf_iterate_ctx(ctx, perf_event_addr_filters_exec,
+ NULL, true);
+ }
+ rcu_read_unlock();
}
- rcu_read_unlock();
-
- perf_event_remove_on_exec();
}
struct remote_output {
