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

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Date:	Tue, 23 Feb 2010 18:15:59 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	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 Tuesday 23 February 2010, Paul E. McKenney wrote:
> We would also need something for initialization of structure fields.
> Does __force work in that case as well?

Yes, it will just need some syntactical sugar to avoid placing __force
in device drivers.

> > If there are cases where it does not work, we need to come up with
> > names for new primitives that just do the assignment or dereference
> > with __force but no actual synchronization.
> 
> Some data structures are shared by RCU and non-RCU code, with struct
> list_head being the most prominent example.  Making the "next" pointer
> as __rcu might be OK, but there are a -lot- of non-RCU uses of struct
> list_head.  Would we really want to introduce rcu_dereference() to all
> non-RCU list-traversal primitives, or do we need to do something else?

I've just started an experimental implementation and got stuck at list rcu.
The two to deal with it that I can see are
- ignore list-rcu for now, and make all include/linux/rculist.h __force the
  problem to be ignored.
- introduce a new struct rcu_list_head that needs to be used for list rcu.

A nicer option might be if sparse would let you write
'struct list_head __rcu head' and interpret that as having the pointers
inside it annotated as __rcu.

	Arnd
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