linux-kernel - Re: [PATCH v3 1/6] rtmutex: Deboost before waking up the top waiter

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Date:	Mon, 18 Apr 2016 10:23:33 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Xunlei Pang <xlpang@...hat.com>
cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@....com>,
	Ingo Molnar <mingo@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v3 1/6] rtmutex: Deboost before waking up the top
 waiter

On Thu, 14 Apr 2016, Xunlei Pang wrote:
> We should deboost before waking the high-prio task such that
> we don't run two tasks with the 'same' priority.

No. This is fundamentaly broken.
 
T1 (prio 0)	lock(X)

-->	 	preemption

T2 (prio 10)	lock(X)
   	 	boost(T1)
		schedule()

T1 (prio 10)	unlock(X)
   	 	deboost()
   (prio 0) 

-->		preemption

T3 (prio 5)	....

Classic priority inversion enabled by a mechanism to avoid it. Brilliant
stuff.

Thanks,

	tglx