linux-kernel - Re: [PATCH RFC tip/core/rcu 7/7] rcu: Quiet RCU-lockdep warnings involving interrupt disabling

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Date:	Tue, 6 Dec 2011 08:04:55 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Yong Zhang <yong.zhang0@...il.com>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	patches@...aro.org
Subject: Re: [PATCH RFC tip/core/rcu 7/7] rcu: Quiet RCU-lockdep warnings
 involving interrupt disabling

On Tue, Dec 06, 2011 at 07:26:45AM -0500, Steven Rostedt wrote:
> On Tue, 2011-12-06 at 11:32 +0100, Peter Zijlstra wrote:
> 
> > > > 
> > > > > Maybe we could teach might_sleep() about this special case?
> > > > 
> > > > Sounds horrid.
> > > 
> > > Maybe, any alternative?
> > 
> > Maybe someone explain this mess first?
> 
> Me too, because this looks like a hack that's just like a lie. The first
> lie to be said gets you out of a bit of trouble, but then something else
> happens based on that lie, in which you need to make another bigger lie.
> This new lie affects more people and requires new more ingenious lies to
> control the chaos. But eventually the lies required to keep everything
> going become so overwhelming that it all blows up in your face and you
> end up looking like a jackass.
> 
> Why is rcu using rt_mutex_lock() in strange ways? It's lying about its
> use. And now this patch is the bigger lie to get around the issues of
> the first lie. Eventually this code will continue to expand largely
> based on these lies and will explode in our faces, and I feel sorry for
> the poor jackass that needs to fix it.
> 
> Perhaps the real answer is that we need to create an API for priority
> inheritance, that things like RCU could use. Attach a task that another
> task requires to finish something and boost the priority of that task.
> Maybe even completions could use such a thing?

I would be OK with that -- that was in fact the approach I was taking
when I was advised to use mutexes instead.  ;-)

							Thanx, Paul

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