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

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Date:   Wed, 28 Sep 2016 11:24:13 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     mingo@...nel.org, tglx@...utronix.de, juri.lelli@....com,
        rostedt@...dmis.org, xlpang@...hat.com,
        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 Wed, Sep 28, 2016 at 11:07:40AM +0200, Sebastian Andrzej Siewior wrote:
> On 2016-09-26 14:32:14 [+0200], Peter Zijlstra 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);
> 
> This breaks -RT. Before that spin_unlock() you do a preempt_disable()
> which means you had one spinlock with enabled preemption and now you get
> one unlock with disabled preemption. And this breaks migrate_disable() /
> enable (because we take the fast path in the in_atomic() case).

Oh crud, the hb lock is not raw :/