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

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Date:	Sat, 9 Apr 2016 15:29:35 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	xlpang@...hat.com
Cc:	Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
	Juri Lelli <juri.lelli@....com>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] sched/deadline/rtmutex: Fix a PI crash for deadline tasks

On Sat, Apr 09, 2016 at 11:25:39AM +0800, Xunlei Pang wrote:

> > In any case, I just realized we do not in fact provide this guarantee
> > (of pointing to a blocked task) that needs a bit more work.
> 
> Current patch calls rt_mutex_adjust_prio() before wake_up_q() the
> wakee, at that moment the wakee has been removed by
> rt_mutex_slowunlock()->mark_wakeup_next_waiter(), from current's
> pi_waiters, rt_mutex_adjust_prio() won't see this wakee, so I think
> this should not be problem.

No, any wakeup after mark_wakeup_next_waiter() will make the task run.
And since we must always consider spurious wakeups, we cannot rely on us
(eg. our wake_up_q call) being the one and only.

Therefore it is possible and the only thing that stands between us and
doom is the fact that the wake_q stuff holds a task reference.

But we cannot guarantee that the task we have a pointer to is in fact
blocked.