linux-kernel - Q: perf_event && event->owner

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Message-ID: <20101108145754.GB3434@redhat.com>
Date:	Mon, 8 Nov 2010 15:57:54 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Alan Stern <stern@...land.harvard.edu>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>, Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Prasad <prasad@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...hat.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Q: perf_event && event->owner

Another thing I can't understand, event->owner/owner_entry.

Say, some thread calls sys_perf_event_open() and creates the event.
It becomes its owner. Now this thread exits, but fd/event are still
here, and event->owner refers to the dead task_struct.

ptrace looks even more strange. Debugger can attach the breakpoint
to the tracee and then exit/detach. ->ptrace_bps events still point
to the same (may be dead) task. Even if another debugger attaches
and reuses these events.

And for what? Afaics, this is only used by PR_TASK_PERF_EVENTS_xxABLE.
Looks like, tools/perf/ used prctl() in the past. Perhaps this API
can die now and we can kill ->owner/owner_entry?

Oleg.

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