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

From: Peter Zijlstra <peterz@infradead.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Alex Shi <alex.shi@linaro.org>,
	mingo@redhat.com, corbet@lwn.net,
	"open list:LOCKING PRIMITIVES" <linux-kernel@vger.kernel.org>,
	Sebastian Siewior <bigeasy@linutronix.de>,
	Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH 2/3] rtmutex: deboost priority conditionally when rt-mutex unlock
Date: Thu, 13 Apr 2017 18:51:42 +0200	[thread overview]
Message-ID: <20170413165142.7g2xsnklhjpkstx2@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20170413124014.63177422@gandalf.local.home>

On Thu, Apr 13, 2017 at 12:40:14PM -0400, Steven Rostedt wrote:
> On Thu, 13 Apr 2017 18:21:13 +0200
> Peter Zijlstra <peterz@infradead.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@infradead.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?

Yep.