Re: [patch 3/14] x86, ptrace, bts: stop bts tracing early indo_exit

[Markus Metzger]

On Fri, 2009-03-27 at 18:24 +0100, Oleg Nesterov wrote:
> On 03/27, Metzger, Markus T wrote:
> >
> > >-----Original Message-----
> > >From: Oleg Nesterov [mailto:oleg@xxxxxxxxxx]
> > >Sent: Friday, March 27, 2009 3:35 PM
> >
> > >> +static void ptrace_bts_exit(void)
> > >> +{
> > >> +	if (unlikely(!list_empty(&current->ptraced)))
> > >> +		ptrace_bts_exit_tracer();
> > >> +
> > >> +	if (unlikely(current->bts))
> > >> +		ptrace_bts_exit_tracee();
> > >> +}
> > >
> > >Could you explain why do we need ptrace_bts_exit_tracee() ?
> > >
> > >If current is traced, the tracer should do ptrace_bts_release()
> > >eventually, no?
> >
> > If current is traced and exits, it may be reaped by another thread that is not
> > the tracer (that's actually your example you made in an earlier thread to
> > describe the race between a normal detach and an exiting tracee).
> >
> > The ptrace_unlink() call to detach the tracer is executed with irq's disabled.
> > I need irq's enabled (see the other discussion, to wait for the traced task).
> 
> OK,
> 
> > Therefore, I have the tracee disable bts tracing itself when it exits.
> >
> >
> > >And if we really need to do ptrace_bts_exit_tracee(), then
> > >"if (unlikely(current->bts))" check is racy. The tracer
> > >can do PTRACE_BTS_CONFIG right after the check.
> >
> > The ptrace system call to do this would require the tracee to be stopped.
> 
> Yes, but this doesn't matter.
> 
> The tracer starts ptrace(PTRACE_BTS_CONFIG) and stops the tracee.
> But when the tracer calls ptrace_bts_config() the tracee can be already
> killed, and it can exit and bypass ptrace_bts_exit_tracee().

Thanks for your review! This is really helpful.


This exit race is a nasty thing.
I think I need to either prevent a task from running while I change the
tracing configuration, or have the tracee execute all the setup/cleanup
code.

The former looks more attractive to me. I would expect that there is
some infrastructure for that. I asked for pointers in a previous email,
already.


thanks and regards,
markus.

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