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Date: Tue, 10 Mar 2015 13:37:40 +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: <20150310123740.GA2896@worktop.programming.kicks-ass.net>
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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.