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Date:	Sun, 23 Mar 2014 12:54:17 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Monam Agarwal <monamagarwal123@...il.com>, davem@...emloft.net,
	jasowang@...hat.com, xemul@...allels.com, wuzhy@...ux.vnet.ibm.com,
	therbert@...gle.com, yamato@...hat.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, paulmck@...ux.vnet.ibm.com
Subject: Re: [PATCH] drivers/net: Use RCU_INIT_POINTER(x, NULL) in tun.c

On Sun, 2014-03-23 at 21:41 +0200, Michael S. Tsirkin wrote:

> The rcu_assign_pointer() ensures that the initialization of a structure       
> is carried out before storing a pointer to that structure. 
> In the case of the NULL pointer, there is no structure to initialize,
> so we can safely drop smp_wmb in this case.
> 
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> 
> --
> 
> Lightly tested.
> v is evaluated twice here but that should be ok since this
> only happens when v is a constant, so evaluating it should
> have no side effects.
> Paul, what do you think?
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 72bf3a0..d33c9ec 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -587,7 +587,8 @@ static inline void rcu_preempt_sleep_check(void)
>   */
>  #define rcu_assign_pointer(p, v) \
>  	do { \
> -		smp_wmb(); \
> +		if (!__builtin_constant_p(v) || (v)) \
> +			smp_wmb(); \
>  		ACCESS_ONCE(p) = RCU_INITIALIZER(v); \
>  	} while (0)
>  

Yeah, I suggest you read d322f45ceed525daa changelog ;)


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