Re: perf_buffer.event_list is not RCU-safe?

[Oleg Nesterov <oleg@redhat.com>]

On 04/07, Peter Zijlstra wrote:
>
> On Tue, Apr 06, 2021 at 07:43:53PM +0200, Oleg Nesterov wrote:
> > On 04/06, Oleg Nesterov wrote:
> > >
> > > perf_mmap_close() was added by 9bb5d40cd93c9 ("perf: Fix mmap() accounting hole")
> >
> > I meant perf_mmap_close() -> put_event()
> >
> > > and this commit doesn't look right anyway
> >
> > It seems there is another problem or I am totally confused. I do not
> > understand why can we use list_for_each_entry_rcu(event, rb->event_list)
> > if this can race with perf_event_set_output(event) which can move "event"
> > to another list, in this case list_for_each_entry_rcu() can loop forever.
> >
> > perf_mmap_close() even mentions this race and restarts the iteration to
> > avoid it but I don't think this is enough,
> >
> > 	rcu_read_lock();
> > 	list_for_each_entry_rcu(event, &rb->event_list, rb_entry) {
> > 		if (!atomic_long_inc_not_zero(&event->refcount)) {
> > 			/*
> > 			 * This event is en-route to free_event() which will
> > 			 * detach it and remove it from the list.
> > 			 */
> > 			continue;
> > 		}
> >
> > just suppose that "this event" is moved to another list first and after
> > that it goes away so that atomic_long_inc_not_zero() fails; in this case
> > the next iteration will play with event->rb_entry.next, and this is not
> > necessarily "struct perf_event", it can can be "list_head event_list".
>
> We observe an RCU GP in ring_buffer_attach(), between detach and attach,
> no?

Aaah yes, I didn't notice cond_synchronize_rcu() in ring_buffer_attach().

Thanks!

Oleg.