Re: [PATCH v2] perf: Synchronously cleanup child events

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Tue, Jan 19, 2016 at 09:05:58PM +0100, Peter Zijlstra wrote:
> On Tue, Jan 19, 2016 at 05:12:34PM +0200, Alexander Shishkin wrote:
> 
> > +static void __put_event(struct perf_event *event)
> >  {
> >  	struct perf_event_context *ctx;
> >  
> >  	if (!is_kernel_event(event))
> >  		perf_remove_from_owner(event);
> >  
> 
> > +int perf_event_release_kernel(struct perf_event *event)
> >  {
> > +	struct perf_event *child, *tmp;
> > +	LIST_HEAD(child_list);
> >  
> > +	if (!is_kernel_event(event))
> > +		perf_remove_from_owner(event);
> >  
> > +	event->owner = NULL;
> >  
> > +retry:
> 
> 	<snip>
> 
> > +	/*
> > +	 * If this is the last reference, we're done here, otherwise
> > +	 * we must have raced with inherit_event(), in which case, repeat
> > +	 */
> > +	if (!put_event_last(event))
> > +		goto retry;
> >  
> > +	return 0;
> > +}
> 
> So I think there's a number of problems still :-(
> 
> I all starts with having two perf_remove_from_owner() calls (as I
> mentioned on IRC), this doesn't make sense.
> 
> I think the moment you close the file and userspace looses control over
> it, we should drop the owner bit, which is exactly the one
> remove_from_owner in perf_release().
> 
> If, for some magical reason, the event lives on after that (and we'll
> get to that), it should live on owner-less.
> 
> Now, assume someone has such a magical reference, then our
> put_event_last() goto again loop will never terminate, this seems like a
> bad thing.
> 
> The most obvious place that generates such magical references would be
> the bpf arraymap doing perf_event_get() on things. There are a few other
> places that take temp references (perf_mmap_close), but those are
> 'short' lived and while ugly will not cause massive grief. The BPF one
> OTOH is a real problem here.
> 
> And looking at the BPF stuff, that code seems to assume
> perf_event_kernel_release() := put_event(), so this patch breaks that
> too.
> 
> 
> Alexei, is there a reason the arraymap stuff needs a perf event ref as
> opposed to a file ref? I'm forever a little confused on how perf<->bpf
> works.

A file ref will not work, since user space could have closed that
perf_event file to avoid unnecessary FDs.
Program only need the stable pointer to 'struct perf_event' which
it will use while running.
At the end it will call perf_event_kernel_release() which
is == put_event().
It was the case that 'perf_events' were normal refcnt-ed structures
and the last guy frees it.
This put_event_last() logic definitely looks problematic.
There are no ordering guarantees.
User space may close FD, while struct perf_event is still alive.
The loop around perf_event_last() looks buggy.
I'm obviously missing the main goal of this patch.