linux-kernel - Re: [PATCH -v2 1/9] rtmutex: Deboost before waking up the top waiter

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Date:   Mon, 26 Sep 2016 17:22:28 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     mingo@...nel.org, tglx@...utronix.de, juri.lelli@....com,
        xlpang@...hat.com, bigeasy@...utronix.de,
        linux-kernel@...r.kernel.org, mathieu.desnoyers@...icios.com,
        jdesfossez@...icios.com, bristot@...hat.com,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH -v2 1/9] rtmutex: Deboost before waking up the top waiter

On Mon, Sep 26, 2016 at 11:15:11AM -0400, Steven Rostedt wrote:
> On Mon, 26 Sep 2016 14:32:14 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:
> > 
> > --- a/kernel/futex.c
> > +++ b/kernel/futex.c
> > @@ -1374,9 +1374,8 @@ static int wake_futex_pi(u32 __user *uad
> >  	 * scheduled away before the wake up can take place.
> >  	 */
> >  	spin_unlock(&hb->lock);
> > -	wake_up_q(&wake_q);
> > -	if (deboost)
> > -		rt_mutex_adjust_prio(current);
> > +
> > +	rt_mutex_postunlock(&wake_q, deboost);
> 
> Hmm...
> 
> >  
> >  	return 0;
> >  }
> > --- a/kernel/locking/rtmutex.c
> > +++ b/kernel/locking/rtmutex.c
> > @@ -1325,6 +1308,16 @@ static bool __sched rt_mutex_slowunlock(
> >  	 */
> >  	mark_wakeup_next_waiter(wake_q, lock);
> >  
> > +	/*
> > +	 * We should deboost before waking the top waiter task such that
> > +	 * we don't run two tasks with the 'same' priority. This however
> > +	 * can lead to prio-inversion if we would get preempted after
> > +	 * the deboost but before waking our high-prio task, hence the
> > +	 * preempt_disable before unlock. Pairs with preempt_enable() in
> > +	 * rt_mutex_postunlock();
> 
> There's a preempt_enable() in rt_mutex_postunlock()? Does
> wake_futex_pi() know that?
> 

Not sure I see your point. rt_mutex_futex_unlock() calls
rt_mutex_slowunlock() which does the preempt_disable(), we then pass the
return of that into deboost, which we pass into rt_mutex_postunlock()
and everything should be balanced.