Messages in this thread | | | Date | Tue, 15 Jul 2014 11:11:04 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 2/5] perf: Destroy event's children on task exit |
| |
On Mon, Jul 14, 2014 at 10:18:54PM +0200, Jiri Olsa wrote: > On Mon, Jul 14, 2014 at 01:18:33PM +0200, Peter Zijlstra wrote: > > On Fri, Jul 11, 2014 at 01:56:19PM +0200, Jiri Olsa wrote: > > > From: Jiri Olsa <jolsa@redhat.com>
> > > +static void perf_event_exit_children(struct perf_event *parent) > > > +{ > > > + struct perf_event *child, *tmp; > > > + > > > + mutex_lock(&parent->child_mutex); > > > + list_for_each_entry_safe(child, tmp, &parent->child_list, > > > + child_list) { > > > + struct perf_event_context *child_ctx = child->ctx; > > > + > > > + /* > > > + * Child events got removed from child_list under > > > + * child_mutex and then freed. So it's safe to access > > > + * childs context in here, because the child holds > > > + * context ref. > > > + */ > > > + mutex_lock(&child_ctx->mutex); > > > + perf_remove_from_context(child, true); > > > + mutex_unlock(&child_ctx->mutex); > > > + > > > + list_del_init(&child->child_list); > > > + put_event(parent); > > > + free_event(child); > > > + } > > > + mutex_unlock(&parent->child_mutex); > > > +}
> > I don't think this is correct, perf_event_init_context() can come in > > concurrently and the first place it runs into ->child_mutex is after its > > already allocated and created the (first) child event. > > just noticed this.. I'm working on the other version we decide, but FWIW > there's also mutex_lock(&child_ctx->mutex); before removing the context, > that should protect it against perf_event_init_context call
Oh, more fail :-)
You have:
perf_event::child_mutex perf_event_context::mutex
The existing code has:
perf_event_context::mutex perf_event::child_context
See for example:
perf_event_init_context() mutex_lock(&parent_ctx->mutex) inherit_task_group() inherit_group() inherit_event() mutex_lock(&parent_event->child_mutex)
and
perf_event_for_each() mutex_lock(&ctx->mutex) perf_event_for_each_child() mutex_lock(&event->child_mutex)
So the patch creates an AB-BA deadlock. [unhandled content-type:application/pgp-signature] | |