linux-kernel - Re: [PATCH tip/core/rcu 0/21] v6 add lockdep-based diagnostics to rcu

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Message-ID: <20100223132815.GA6873@linux.vnet.ibm.com>
Date:	Tue, 23 Feb 2010 05:28:15 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	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,
	dvhltc@...ibm.com, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com
Subject: Re: [PATCH tip/core/rcu 0/21] v6 add lockdep-based diagnostics to
	rcu_dereference()

On Tue, Feb 23, 2010 at 01:59:07PM +0100, Arnd Bergmann wrote:
> On Tuesday 23 February 2010, Paul E. McKenney wrote:
> > This patch series adds lockdep-based checking to the rcu_dereference()
> > primitive in order to flag misuses of RCU.
> 
> While I haven't looked much at this series, I've been thinking about
> adding static diagnostics for rcu_dereference misuse, in the form of
> an __rcu address space qualifier for pointers. Such a patch would
> obviously conflict with this series, so I'd wait for yours to go
> in first, but maybe you like the idea enough to do it yourself ;-).
> 
> The observation is that all accesses to an RCU protected pointer
> are either through rcu_dereference, rcu_assign_pointer or one of their
> variants. so it should be possible to add a new address space like we
> have for __iomem, __user and soon __percpu and have sparse check that
> we use RCU consistently on pointers that need it.

That is orthogonal to the current series, but sounds like a very good
idea, give or take how to handle initialization accesses and the like.

Your approach would verify that a given RCU-protected pointer is
consistently handled by rcu_dereference() and presumably also
rcu_assign_pointer().  This patch series instead verifies that a given
rcu_dereference() is protected by the proper flavor of RCU or the
correct lock or whatever else.

							Thanx, Paul
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