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 09:16:05 -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 11:56:42AM -0500, Steven Rostedt wrote:
> On Tue, 2011-12-06 at 08:04 -0800, Paul E. McKenney wrote:
> 
> > > 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.  ;-)
> 
> Maybe we should rethink it. Using the makeshift mutex looks to be a
> short term hack. But if we are starting to build on it, it will end up
> being a horrible design, based off of a hack.
> 
> A mutex is to provide mutual exclusion. If we start bastardizing it to
> do other things, it will become unmaintainable. I dare say that it's
> close to unmaintainable now ;)
> 
> If we create a new API to handle inheritance, then perhaps it could be
> used for other things like workqueues and completions (in -rt only).

Tough choice between yours and Peter's suggestion...

1.	Re-introduce ugly races by eliminating the mutex.

2.	Possibly have to deal with a new spate of lockdep-RCU splats.

Decisions, decisions!  ;-)

I suppose that one approach is to start with Peter's approach,
possibly adapting lockdep to explicitly check -- with an exception for
srcu_read_lock_raw(), of course.  If lockdep-RCU splats rain down too
hard, then perhaps the explicit priority inheritance would be one
potential umbrella.

Thoughts?

							Thanx, Paul

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