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

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Date:	Mon, 18 Apr 2016 16:44:58 +0800
From:	Xunlei Pang <xpang@...hat.com>
To:	Thomas Gleixner <tglx@...utronix.de>
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 2016/04/18 at 16:23, Thomas Gleixner wrote:
> 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)

We add a preempt_disable() before deboost to avoid the breakage,
there's also some comment about this in the patch's code.

Regards,
Xunlei

>    	 	deboost()
>    (prio 0) 
>
> -->		preemption
>
> T3 (prio 5)	....
>
> Classic priority inversion enabled by a mechanism to avoid it. Brilliant
> stuff.
>
> Thanks,
>
> 	tglx