netdev - Re: [PATCH] fib_trie: rcu_assign

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Message-ID: <20080212012741.GD29254@linux.vnet.ibm.com>
Date:	Mon, 11 Feb 2008 17:27:41 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Stephen Hemminger <shemminger@...tta.com>
Cc:	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH] fib_trie: rcu_assign_pointer warning fix

On Mon, Feb 11, 2008 at 04:59:54PM -0800, Stephen Hemminger wrote:
> Eliminate warnings when rcu_assign_pointer is used with unsigned long.
> It is reasonable to use RCU with non-pointer values so allow it for general
> use.  Add a comment to explain the if test.

Good catch!!!  (An apologies for the hassle!!!)

Acked-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

> Signed-off-by: Stephen Hemminger <shemminger@...tta.com>
> ---
>  include/linux/rcupdate.h |   13 +++++++------
>  1 files changed, 7 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 37a642c..c44ac87 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -172,14 +172,15 @@ struct rcu_head {
>   * structure after the pointer assignment.  More importantly, this
>   * call documents which pointers will be dereferenced by RCU read-side
>   * code.
> + *
> + * If value is the NULL (constant 0), then no barrier is needed.
>   */
> 
> -#define rcu_assign_pointer(p, v) \
> -	({ \
> -		if (!__builtin_constant_p(v) || \
> -		    ((v) != NULL)) \
> -			smp_wmb(); \
> -		(p) = (v); \
> +#define rcu_assign_pointer(p, v)			\
> +	({						\
> +		if (!(__builtin_constant_p(v) && v))	\
> +			smp_wmb();			\
> +		(p) = (v);				\
>  	})
> 
>  /**
> -- 
> 1.5.3.8
> 
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