Re: [PATCH v3 1/6] rtmutex: Deboost before waking up the top waiter
From: Peter Zijlstra
Date: Wed Apr 20 2016 - 09:11:05 EST
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 :(
> > 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..