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Subject: Re: [PATCH tip/core/rcu 1/3] ftrace: replace
 read_barrier_depends() with rcu_dereference_raw()
From: Steven Rostedt <rostedt@goodmis.org>
Reply-To: rostedt@goodmis.org
To: Josh Triplett <josh@joshtriplett.org>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
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Date: Fri, 05 Mar 2010 20:57:49 -0500
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On Fri, 2010-03-05 at 17:45 -0800, Josh Triplett wrote:

> > #define rcu_assign_pointer(p, v) \
> > 	({ \
> > 		if (!__builtin_constant_p(v) || \
> > 		    ((v) != NULL)) \
> > 			smp_wmb(); \
> > 		(p) = (v); \
> > 	})
> > 
> > My question is, why that crazy if? The only time that will fail is if we
> > are assigning the constant NULL to p. What makes NULL so important here?
> > Can't there be a case when assigning NULL to p will require that wmb()?
> 
> The barrier ensures that the reader can't see the new p and the old
> *p.  Since you can't look at *NULL, that concern doesn't apply.

Thanks for the explanation.

Question 2)

Then why the !__builtin_constant_p(v)?

If v is NULL, then the same should apply even if it is not a constant?
What am I missing?

-- Steve