linux-kernel - Re: [PATCH 2/3] locking: ww_mutex: Allow to use rt

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Message-ID: <20150310152826.GE2896@worktop.programming.kicks-ass.net>
Date:	Tue, 10 Mar 2015 16:28:26 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Maarten Lankhorst <maarten.lankhorst@...onical.com>
Cc:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>
Subject: Re: [PATCH 2/3] locking: ww_mutex: Allow to use rt_mutex instead of
 mutex for the baselock

On Tue, Mar 10, 2015 at 03:10:46PM +0100, Maarten Lankhorst wrote:
> Op 10-03-15 om 13:37 schreef Peter Zijlstra:

> > So IIRC this is the function that checks who gets wounded (and gets to
> > do the whole retry thing), right?
> >
> > So for the RT case, I think we should extend it to not (primarily) be a
> > FIFO thing, but also consider the priority of the tasks involved.
> >
> > Maybe a little something like:
> >
> > 	if (hold_ctx->task->prio < ctx->task->prio)
> > 		return -EDEADLOCK;
> >
> > before the timestamp check; although I suppose we should also add a
> > deadline test in case both prios are -1.

> I think that's useful but if you implement -EDEADLK based on thread
> priority, any boosted thread should receive -EDEADLK when it tries to
> acquire a new lock in the same context, to force it to back off..

But _only_ if the boost comes from inside the ww_ctx, right?

Yeah, implementing that might be tricky, maybe we can keep it as a note
for now ;)
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