linux-kernel - Re: Q: perf_event && task->ptrace

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Message-ID: <20110119153746.GA32563@redhat.com>
Date:	Wed, 19 Jan 2011 16:37:46 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Arnaldo Carvalho de Melo <acme@...hat.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>,
	linux-kernel@...r.kernel.org
Subject: Re: Q: perf_event && task->ptrace_bps[]

On 01/18, Frederic Weisbecker wrote:
>
> On Mon, Jan 17, 2011 at 09:34:59PM +0100, Oleg Nesterov wrote:
> >
> > Any idea how to fix this cleanly? May be we can reuse perf_event_mutex,
> > but this looks soooo ugly. And do_exit()->flush_ptrace_hw_breakpoint()
> > has the strange "FIXME:" comment which doesn't help me to understand
> > what can we do.
>
> Yeah forget about the FIXME, it's a stale thing I need to remove.

OK, good.

> > Probably the best fix is to change this code so that the tracer owns
> > ->ptrace_bps[], not the tracee. But this is not trivial, and needs a
> > lot of changes in ptrace code.
>
> How much complicated would it be?

The problem is, ptrace_detach/release_task can't sleep currently.
The necessary changes are nasty.

> Because I see three solutions to solve this:
>
> - Have a mutex inside thread->ptrace_bps. The contention must be
> rare and only concern ptrace and tracee exit. That's the simplest.

I think we can reuse perf_event_mutex for this. Not very good too,
but simple. But this depends on what can we do under this mutex...

I am going to report a couple more bugs (at least, it looks like
a bug when I am trying to understand the code ;), probably they
should be fixed first.

Oleg.

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