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

From Alexei Starovoitov <alexei.starovoitov@gmail.com>
Newsgroups linux.kernel
Subject Re: [PATCH v2] perf: Synchronously cleanup child events
Date 2016-01-19 23:00 +0100
Message-ID <qSMtI-5dk-13@gated-at.bofh.it> (permalink)
References <qSji2-1ZW-23@gated-at.bofh.it> <qSGeC-1c0-13@gated-at.bofh.it> <qSKLh-4he-33@gated-at.bofh.it>
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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.

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Thread

[PATCH] perf: Cleanup user's child events Alexander Shishkin <alexander.shishkin@linux.intel.com> - 2016-01-15 12:30 +0100
  Re: [PATCH] perf: Cleanup user's child events Peter Zijlstra <peterz@infradead.org> - 2016-01-15 14:00 +0100
    Re: [PATCH] perf: Cleanup user's child events Alexander Shishkin <alexander.shishkin@linux.intel.com> - 2016-01-15 14:10 +0100
      Re: [PATCH] perf: Cleanup user's child events Peter Zijlstra <peterz@infradead.org> - 2016-01-15 14:10 +0100
        [PATCH] perf: Synchronously cleanup child events Alexander Shishkin <alexander.shishkin@linux.intel.com> - 2016-01-15 15:10 +0100
          Re: [PATCH] perf: Synchronously cleanup child events Peter Zijlstra <peterz@infradead.org> - 2016-01-15 19:00 +0100
            Re: [PATCH] perf: Synchronously cleanup child events Alexander Shishkin <alexander.shishkin@linux.intel.com> - 2016-01-18 13:20 +0100
            Re: [PATCH] perf: Synchronously cleanup child events Alexander Shishkin <alexander.shishkin@linux.intel.com> - 2016-01-18 13:40 +0100
              Re: [PATCH] perf: Synchronously cleanup child events Peter Zijlstra <peterz@infradead.org> - 2016-01-18 15:50 +0100
                [PATCH v2] perf: Synchronously cleanup child events Alexander Shishkin <alexander.shishkin@linux.intel.com> - 2016-01-19 16:20 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Peter Zijlstra <peterz@infradead.org> - 2016-01-19 21:10 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Peter Zijlstra <peterz@infradead.org> - 2016-01-19 21:10 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Alexei Starovoitov <alexei.starovoitov@gmail.com> - 2016-01-19 23:00 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Peter Zijlstra <peterz@infradead.org> - 2016-01-20 09:40 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Alexei Starovoitov <alexei.starovoitov@gmail.com> - 2016-01-21 06:00 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Alexander Shishkin <alexander.shishkin@linux.intel.com> - 2016-01-20 08:10 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Peter Zijlstra <peterz@infradead.org> - 2016-01-20 09:10 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Alexander Shishkin <alexander.shishkin@linux.intel.com> - 2016-01-22 12:40 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Peter Zijlstra <peterz@infradead.org> - 2016-01-22 13:20 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Peter Zijlstra <peterz@infradead.org> - 2016-01-22 13:40 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Alexei Starovoitov <alexei.starovoitov@gmail.com> - 2016-01-22 20:50 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Peter Zijlstra <peterz@infradead.org> - 2016-01-25 12:50 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Peter Zijlstra <peterz@infradead.org> - 2016-01-25 16:00 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Peter Zijlstra <peterz@infradead.org> - 2016-01-25 22:10 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Alexei Starovoitov <alexei.starovoitov@gmail.com> - 2016-01-26 06:10 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Peter Zijlstra <peterz@infradead.org> - 2016-01-26 17:20 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Peter Zijlstra <peterz@infradead.org> - 2016-01-26 18:30 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Alexei Starovoitov <alexei.starovoitov@gmail.com> - 2016-01-27 00:40 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Peter Zijlstra <peterz@infradead.org> - 2016-01-27 11:00 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Alexei Starovoitov <alexei.starovoitov@gmail.com> - 2016-01-27 19:00 +0100
          Re: [PATCH] perf: Synchronously cleanup child events Ingo Molnar <mingo@kernel.org> - 2016-01-19 08:50 +0100

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