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Reply-To: xlpang@redhat.com
Subject: Re: [PATCH v3 1/6] rtmutex: Deboost before waking up the top waiter
References: <1460633827-345-1-git-send-email-xlpang@redhat.com>
 <1460633827-345-2-git-send-email-xlpang@redhat.com>
 <alpine.DEB.2.11.1604181018080.3941@nanos>
To: Thomas Gleixner <tglx@linutronix.de>
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>
From: Xunlei Pang <xpang@redhat.com>
Message-ID: <57149E8A.6060701@redhat.com>
Date: Mon, 18 Apr 2016 16:44:58 +0800
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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