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

From: Thomas Gleixner
Date: Wed Apr 20 2016 - 08:45:10 EST

On Wed, 20 Apr 2016, Peter Zijlstra wrote:
> On Mon, Apr 18, 2016 at 11:02:28AM +0200, Thomas Gleixner wrote:
> > On Mon, 18 Apr 2016, Xunlei Pang wrote:
> > > We add a preempt_disable() before deboost to avoid the breakage,
> > > there's also some comment about this in the patch's code.
> >
> > So the changelog is useless and misleading. Neither does it explain what's
> > wrong with having two tasks with the same priority in running state.
> 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. That really gets interesting with bandwith inheritance ....