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

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Date:	Fri, 27 Mar 2009 15:34:19 +0000
From:	"Metzger, Markus T" <markus.t.metzger@...el.com>
To:	Oleg Nesterov <oleg@...hat.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

>-----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).

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.


regards,
markus.

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