linux-kernel - Re: [PATCH 1/1] do_exit(): Solve possibility of BUG() due to race with try_to_wake

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Message-ID: <20140902173910.GF27892@worktop.ger.corp.intel.com>
Date:	Tue, 2 Sep 2014 19:39:10 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Kautuk Consul <consul.kautuk@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.cz>,
	David Rientjes <rientjes@...gle.com>,
	Ionut Alexa <ionut.m.alexa@...il.com>,
	Guillaume Morin <guillaume@...infr.org>,
	linux-kernel@...r.kernel.org, Kirill Tkhai <tkhai@...dex.ru>
Subject: Re: [PATCH 1/1] do_exit(): Solve possibility of BUG() due to race
 with try_to_wake_up()

On Tue, Sep 02, 2014 at 06:47:14PM +0200, Oleg Nesterov wrote:

> But since I already wrote v2 yesterday, let me show it anyway. Perhaps
> you will notice something wrong immediately...
> 
> So, once again, this patch adds the ugly "goto" into schedule(). OTOH,
> it removes the ugly spin_unlock_wait(pi_lock).

But schedule() is called _far_ more often than exit(). It would be
really good not to have to do that. 

> TASK_DEAD can die. The only valid user is schedule_debug(), trivial to
> change. The usage of TASK_DEAD in task_numa_fault() is wrong in any case.
> 
> In fact, I think that the next change can change exit_schedule() to use
> PREEMPT_ACTIVE, and then we can simply remove the TASK_DEAD check in
> schedule_debug().

So you worry about concurrent wakeups vs setting TASK_DEAD and thereby
loosing it, right?

Would not something like:

	spin_lock_irq(&current->pi_lock);
	__set_current_state(TASK_DEAD);
	spin_unlock_irq(&current->pi_lock);

Not be race free and similarly expensive to the smp_mb() we have there
now?

> -	BUG();
> -	/* Avoid "noreturn function does return".  */
> -	for (;;)
> -		cpu_relax();	/* For when BUG is null */


> +void exit_schedule(void)
> +{
> +	current->state = TASK_DEAD; /* TODO: kill TASK_DEAD altogether */
> +	task_rq(current)->prev_dead = true;
> +	__schedule();
> +	BUG();

you lost that for loop.
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