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Date: Tue, 10 Mar 2015 13:39:37 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
        Maarten Lankhorst <maarten.lankhorst@canonical.com>,
        Mike Galbraith <umgwanakikbuti@gmail.com>
Subject: Re: [PATCH 2/3] locking: ww_mutex: Allow to use rt_mutex instead of
 mutex for the baselock
Message-ID: <20150310123937.GB11574@worktop.ger.corp.intel.com>
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On Tue, Mar 10, 2015 at 01:37:40PM +0100, Peter Zijlstra wrote:
> On Fri, Feb 27, 2015 at 05:57:08PM +0100, Sebastian Andrzej Siewior wrote:
> > +static int __sched __mutex_lock_check_stamp(struct rt_mutex *lock,
> > +					    struct ww_acquire_ctx *ctx)
> > +{
> > +#ifdef CONFIG_WW_MUTEX_RTMUTEX
> > +	struct ww_mutex *ww = container_of(lock, struct ww_mutex, base.lock);
> > +	struct ww_acquire_ctx *hold_ctx = ACCESS_ONCE(ww->ctx);
> > +
> > +	if (!hold_ctx)
> > +		return 0;
> > +
> > +	if (unlikely(ctx == hold_ctx))
> > +		return -EALREADY;
> > +
> > +	if (ctx->stamp - hold_ctx->stamp <= LONG_MAX &&
> > +	    (ctx->stamp != hold_ctx->stamp || ctx > hold_ctx)) {
> > +#ifdef CONFIG_DEBUG_MUTEXES
> > +		DEBUG_LOCKS_WARN_ON(ctx->contending_lock);
> > +		ctx->contending_lock = ww;
> > +#endif
> > +		return -EDEADLK;
> > +	}
> > +#endif
> > +	return 0;
> > +}
> 
> 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.

Something like rt_mutex_waiter_less(), we should have the same ordering
as that.