linux-kernel - Re: [PATCH tip/urgent] rcu: add rcu_access_pointer and rcu_dereference

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Date:	Wed, 7 Apr 2010 10:13:42 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	David Howells <dhowells@...hat.com>
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,
	eric.dumazet@...il.com
Subject: Re: [PATCH tip/urgent] rcu: add rcu_access_pointer and
 rcu_dereference_protected

On Wed, Apr 07, 2010 at 06:00:27PM +0100, David Howells wrote:
> Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > The new rcu_access_pointer() primitive is for the case where the pointer
> > is be fetch and not dereferenced.  This primitive may be used without
> > protection, RCU or otherwise, due to the fact that it uses ACCESS_ONCE().
> > ...
> > +#define rcu_access_pointer(p, c) \
> 
> NAK.  This shouldn't have the conditional parameter 'c'.  Given that 'c' (by
> analogy to rcu_dereference_check()) is there to describe the conditions under
> which it's permitted to dereference the pointer, why is that relevant here?
> What is it you're proving?

In some cases, the value of 'c' will indeed be '1', and in those cases,
there needs to be a comment stating why it is OK, similar to those
required for smp_mb() and friends.

In other cases, there will be a reference counter or a "not yet fully
initialized" flag that can (and should) be tested.

							Thanx, Paul
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