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

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Message-ID: <20170413124014.63177422@gandalf.local.home>
Date:   Thu, 13 Apr 2017 12:40:14 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.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, 13 Apr 2017 18:21:13 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> 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.

Because of deadline scheduling?

-- Steve