linux-kernel - Re: [PATCH 1/1] futex: don't spin waiting for PF_EXITING -> PF

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Date:	Fri, 27 Feb 2015 12:54:28 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Darren Hart <darren@...art.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jerome Marchand <jmarchan@...hat.com>,
	Larry Woodman <lwoodman@...hat.com>,
	Mateusz Guzik <mguzik@...hat.com>,
	linux-kernel@...r.kernel.org,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	Pavel Emelyanov <xemul@...allels.com>
Subject: Re: [PATCH 1/1] futex: don't spin waiting for PF_EXITING ->
 PF_EXITPIDONE transition

On Fri, Feb 27, 2015 at 10:52:40AM +0100, Peter Zijlstra wrote:
> On Mon, Feb 16, 2015 at 09:13:36PM +0100, Oleg Nesterov wrote:
> 
> > diff --git a/kernel/futex.c b/kernel/futex.c
> > index b101381..c1104a8 100644
> > +++ b/kernel/futex.c
> > @@ -716,11 +716,13 @@ void exit_pi_state_list(struct task_struct *curr)
> >  
> >  	if (!futex_cmpxchg_enabled)
> >  		return;
> > +
> >  	/*
> > +	 * attach_to_pi_owner() can no longer add the new entry. But
> > +	 * we have to be careful versus waiters unqueueing themselves.
> >  	 */
> > +	curr->flags |= PF_EXITPIDONE;
> > +
> >  	raw_spin_lock_irq(&curr->pi_lock);
> >  	while (!list_empty(head)) {
> >  
> 
> Should we not set PF_EXITPIDONE _inside_ the pi_lock? To properly
> serialize against the below check?

It does not matter, n/m.
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