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

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Message-ID: <20160420131054.GA3430@twins.programming.kicks-ass.net>
Date:	Wed, 20 Apr 2016 15:10:54 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	xlpang@...hat.com, linux-kernel@...r.kernel.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 Wed, Apr 20, 2016 at 02:43:29PM +0200, Thomas Gleixner wrote:
> > So its semantically icky to have the two tasks running off the same
> > state and practically icky when you consider bandwidth inheritance --
> > where the boosted task wants to explicitly modify the state of the
> > booster.
> >
> > In that latter case you really want to unboost before you let the
> > booster run again.
> 
> I understand that. That doesn't make the changelog any better, which mumbles
> about priorities :(

Agreed.

> > However, you noted we need to deal with this case due to the whole
> > optimistic spinning crap anyway :/
> 
> Right, but that's another dimension of madness. Both tasks are on a cpu.

> The reason why we boost the lock holder before spinning is to make
> sure that it does not get preempted by something of medium priority
> before dropping the lock. 

Right; I figured that out pretty quickly, which is why this patch does a
preempt_disable() over the unboost+wakeup.

FWIW, the immediate reason for this patch is that is ensures the new
p->pi_task pointer, points to something that exists.

> That really gets interesting with bandwith inheritance ....

I'm more worried about the optimistic spinning case..