linux-kernel - Re: [patch 3/14] x86, ptrace, bts: stop bts tracing early in do

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Date:	Fri, 27 Mar 2009 18:24:13 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Metzger, Markus T" <markus.t.metzger@...el.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"hpa@...or.com" <hpa@...or.com>,
	"markus.t.metzger@...il.com" <markus.t.metzger@...il.com>,
	"roland@...hat.com" <roland@...hat.com>,
	"eranian@...glemail.com" <eranian@...glemail.com>,
	"Villacis, Juan" <juan.villacis@...el.com>,
	"ak@...ux.jf.intel.com" <ak@...ux.jf.intel.com>
Subject: Re: [patch 3/14] x86, ptrace, bts: stop bts tracing early in do_exit

On 03/27, Metzger, Markus T wrote:
>
> >-----Original Message-----
> >From: Oleg Nesterov [mailto:oleg@...hat.com]
> >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().

Oleg.

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