netdev - Re: [PATCH] net/core: Use RCU_INIT_POINTER(x, NULL) in netpoll.c

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Date:	Mon, 24 Mar 2014 07:11:44 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Monam Agarwal <monamagarwal123@...il.com>
Cc:	davem@...emloft.net, ebiederm@...ssion.com, cwang@...pensource.com,
	kaber@...sh.net, antonio@...hcoding.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net/core: Use RCU_INIT_POINTER(x, NULL) in netpoll.c

On Mon, Mar 24, 2014 at 12:42:46AM +0530, Monam Agarwal wrote:
> This patch replaces rcu_assign_pointer(x, NULL) with RCU_INIT_POINTER(x, NULL)
> 
> The rcu_assign_pointer() ensures that the initialization of a structure       
> is carried out before storing a pointer to that structure. 
> And in the case of the NULL pointer, there is no structure to initialize. 
> So, rcu_assign_pointer(p, NULL) can be safely converted to RCU_INIT_POINTER(p, NULL)
> 
> Signed-off-by: Monam Agarwal <monamagarwal123@...il.com>
No, I don't think this is right. __netpoll_cleanup can be called in parallel
with bottom half access to dev->npinfo.  rcu_assign pointer is safe against
those accesses, but RCU_INIT_POINTER is not.

Neil

> ---
>  net/core/netpoll.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/core/netpoll.c b/net/core/netpoll.c
> index 7291dde..41c4e9c 100644
> --- a/net/core/netpoll.c
> +++ b/net/core/netpoll.c
> @@ -809,7 +809,7 @@ void __netpoll_cleanup(struct netpoll *np)
>  		if (ops->ndo_netpoll_cleanup)
>  			ops->ndo_netpoll_cleanup(np->dev);
>  
> -		rcu_assign_pointer(np->dev->npinfo, NULL);
> +		RCU_INIT_POINTER(np->dev->npinfo, NULL);
>  		call_rcu_bh(&npinfo->rcu, rcu_cleanup_netpoll_info);
>  	}
>  }
> -- 
> 1.7.9.5
> 
> 
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