From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: David Howells <dhowells@redhat.com>
Cc: linux-kernel@vger.kernel.org, mingo@elte.hu,
laijs@cn.fujitsu.com, dipankar@in.ibm.com,
akpm@linux-foundation.org, mathieu.desnoyers@polymtl.ca,
josh@joshtriplett.org, dvhltc@us.ibm.com, niv@us.ibm.com,
tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org,
Valdis.Kletnieks@vt.edu, eric.dumazet@gmail.com
Subject: Re: [PATCH tip/urgent] rcu: add rcu_access_pointer and rcu_dereference_protected
Date: Wed, 7 Apr 2010 10:13:42 -0700 [thread overview]
Message-ID: <20100407171342.GF2481@linux.vnet.ibm.com> (raw)
In-Reply-To: <27310.1270659627@redhat.com>
On Wed, Apr 07, 2010 at 06:00:27PM +0100, David Howells wrote:
> Paul E. McKenney <paulmck@linux.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
next prev parent reply other threads:[~2010-04-07 17:13 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-04-07 16:26 [PATCH tip/urgent] rcu: add rcu_access_pointer and rcu_dereference_protected Paul E. McKenney
2010-04-07 16:38 ` Eric Dumazet
2010-04-07 16:45 ` Mathieu Desnoyers
2010-04-07 17:00 ` David Howells
2010-04-07 17:13 ` Paul E. McKenney [this message]
2010-04-07 17:20 ` David Howells
2010-04-07 23:00 ` Paul E. McKenney
2010-04-08 16:46 ` Paul E. McKenney
2010-04-08 19:04 ` David Howells
2010-04-08 19:53 ` Paul E. McKenney
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