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

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Message-ID: <20100407230009.GK2481@linux.vnet.ibm.com>
Date:	Wed, 7 Apr 2010 16:00:09 -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 06:20:48PM +0100, David Howells wrote:
> Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > In other cases, there will be a reference counter or a "not yet fully
> > initialized" flag that can (and should) be tested.
> 
> Why would you be using rcu_access_pointer() there?  Why wouldn't you be using
> rcu_dereference_protected()?

Excellent question.  I am writing up the documentation now, and will
either (1) have a good use case or (2) remove the condition.

> Also, one other thing:  Should the default versions of these functions make
> some reference to 'c' to prevent compiler warnings?  Should:
> 
> 	#define rcu_dereference_check(p, c)	rcu_dereference_raw(p)
> 
> for example, be:
> 
> 	#define rcu_dereference_check(p, c)	\
> 	({					\
> 		if (1 || !(c))			\
> 			rcu_dereference_raw(p);	\
> 	})
> 
> I'm not sure it's necessary, but it's possible to envisage a situation where
> someone calculates something specifically for use in 'c', which will cause an
> warning from the compiler if c isn't then checked.

I did try this.  The problem is that it breaks the build for non-lockdep
configurations due to the lockdep-check primitives not being defined.  :-(

							Thanx, Paul
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