linux-kernel - Re: hit a KASan bug related to Perf during stress test

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Message-ID: <20161026090332.GA3102@twins.programming.kicks-ass.net>
Date:   Wed, 26 Oct 2016 11:03:32 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     "Ni, BaoleX" <baolex.ni@...el.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "acme@...nel.org" <acme@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "alexander.shishkin@...ux.intel.com" 
        <alexander.shishkin@...ux.intel.com>,
        "Liu, Chuansheng" <chuansheng.liu@...el.com>
Subject: Re: hit a KASan bug related to Perf during stress test

On Tue, Oct 25, 2016 at 04:41:26PM +0200, Oleg Nesterov wrote:
> >
> > So what serialization would close that race? __task_pid_nr_ns() only
> > seems to use RCU nothing more.
> 
> I do not see how can we close this race, we obviously do not want to use
> any locking.
> 
> That is why I tried to suggest
> 
> 	nr = __task_pid_nr_ns(p, type, event->ns);
> 	if (!nr && !is_idle_task(p))
> 		nr = -1;
> 	return nr;
> 
> but this will report -1 if p runs in another namespace, so perhaps we
> can do
> 
> 	nr = __task_pid_nr_ns(p, type, event->ns);
> 	if (!nr && p->exit_state)
> 		// it has already called exit_notify
> 		nr = -1;
> 	return nr;

I think I'm asking how __task_pid_nr_ns() isn't susceptible to this race
;-)