linux-kernel - Re: [RFC][PATCH 2/8] sched/rtmutex/deadline: Fix a PI crash for deadline tasks

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Date:	Tue, 14 Jun 2016 22:28:34 +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, linux-kernel@...r.kernel.org,
	mathieu.desnoyers@...icios.com, jdesfossez@...icios.com,
	bristot@...hat.com, Ingo Molnar <mingo@...hat.com>
Subject: Re: [RFC][PATCH 2/8] sched/rtmutex/deadline: Fix a PI crash for
 deadline tasks

On Tue, Jun 14, 2016 at 02:42:17PM -0400, Steven Rostedt wrote:
> On Tue, 07 Jun 2016 21:56:37 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:
> > +	/*
> > +	 * 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();
> > +	 */
> > +	preempt_disable();
> > +
> 
> This looks like a possible maintenance nightmare. Can we add some more
> comments at the start of the functions that state that
> rt_mutex_slowunlock() calls must be paired with rt_mutex_postunlock()?

Please look at patches 4 and 5 that clean this up.