linux-kernel - Re: [PATCH tip/core/rcu 1/3] ftrace: replace read_barrier_depends() with rcu_dereference

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Date:	Fri, 05 Mar 2010 20:57:49 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, dvhltc@...ibm.com, niv@...ibm.com,
	tglx@...utronix.de, peterz@...radead.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH tip/core/rcu 1/3] ftrace: replace
 read_barrier_depends() with rcu_dereference_raw()

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


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