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