linux-kernel - Re: [PATCH 2/3] rtmutex: deboost priority conditionally when rt-mutex unlock

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Message-ID: <20170413162113.wgtli7v5hp6ersyb@hirez.programming.kicks-ass.net>
Date:   Thu, 13 Apr 2017 18:21:13 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Alex Shi <alex.shi@...aro.org>, mingo@...hat.com, corbet@....net,
        "open list:LOCKING PRIMITIVES" <linux-kernel@...r.kernel.org>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 2/3] rtmutex: deboost priority conditionally when
 rt-mutex unlock

On Thu, Apr 13, 2017 at 12:09:25PM -0400, Steven Rostedt wrote:
> On Thu, 13 Apr 2017 16:39:52 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > On Thu, Apr 13, 2017 at 10:02:53PM +0800, Alex Shi wrote:
> > >  	/*
> > > +	 * 'current' release this lock, so 'current' should be a higher prio
> > > +	 * task than the next top waiter, unless the current prio was gotten
> > > +	 * from this top waiter, iff so, we need to deboost 'current' after
> > > +	 * the lock release.
> > > +	 */
> > > +	if (current->prio == waiter->prio)
> > > +		deboost = true;  
> > 
> > This is wrong.
> 
> The comment is, especially that "iff". What if current and waiter
> happen to have the same priority? Then it too doesn't need to be
> deboosted.

The wrongness is in comparing prio and thinking it means anything.