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

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Message-ID: <27837.1270660848@redhat.com>
Date:	Wed, 07 Apr 2010 18:20:48 +0100
From:	David Howells <dhowells@...hat.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	dhowells@...hat.com, 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

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()?

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.

David
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