Messages in this thread | | | Subject | Re: [PATCH tip/core/rcu 1/3] ftrace: replace read_barrier_depends() with rcu_dereference_raw() | From | Steven Rostedt <> | 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
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