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Subject: Re: Q: perf_event && event->owner
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>,
        Alan Stern <stern@rowland.harvard.edu>,
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Date: Tue, 09 Nov 2010 18:07:49 +0100
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On Tue, 2010-11-09 at 17:58 +0100, Oleg Nesterov wrote:
> On 11/09, Peter Zijlstra wrote:
> >
> > On Tue, 2010-11-09 at 16:57 +0100, Oleg Nesterov wrote:
> > >
> > > If the creator of perf_event dies, nobody can use its ->perf_event_list
> > > anyway. What is the point to keep the reference to the dead task_struct
> > > and preserve this ->perf_event_list?
> >
> > But when the owner dies it will close all its fds, which means it will
> > clear its tsk->perf_event_list, no? (With exception of the case where
> > the fd was passed through a unix-socket to another process).
> 
> fork(), pthread_create(). Only __fput() calls ->release, when the last
> reference to file goes away.

Ah,.. quite so. So how about we explicitly destroy the list when the
task dies?

> And ptrace(), it doesn't use sys_perf_event_open() to create the event.

Right, I guess it uses kernel based things, I guess we could not add
kernel based counters to the list.