Re: [Patch net-next] audit: remove useless synchronize_net()

Re: [Patch net-next] audit: remove useless synchronize_net()

[Richard Guy Briggs]

On 2016-11-29 09:14, Cong Wang wrote:
> netlink kernel socket is protected by refcount, not RCU.
> Its rcv path is neither protected by RCU. So the synchronize_net()
> is just pointless.

If I understand correctly, xfrm_user_net_exit() usage of
RCU_INIT_POINTER() and synchronize_net() is similarly pointless?  Also
net/phonet/socket.c?  I probably modelled things based on the former...

> Cc: Richard Guy Briggs <rgb@redhat.com>
> Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
> ---
>  kernel/audit.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/kernel/audit.c b/kernel/audit.c
> index 92c463d..67b9fbd8 100644
> --- a/kernel/audit.c
> +++ b/kernel/audit.c
> @@ -1172,9 +1172,8 @@ static void __net_exit audit_net_exit(struct net *net)
>  		audit_sock = NULL;
>  	}
>  
> -	RCU_INIT_POINTER(aunet->nlsk, NULL);
> -	synchronize_net();
>  	netlink_kernel_release(sock);
> +	aunet->nlsk = NULL;
>  }
>  
>  static struct pernet_operations audit_net_ops __net_initdata = {
> -- 
> 2.1.0
> 

- RGB

Re: [Patch net-next] audit: remove useless synchronize_net()

[Cong Wang]

On Wed, Nov 30, 2016 at 1:16 AM, Richard Guy Briggs <rgb@redhat.com> wrote:
> On 2016-11-29 09:14, Cong Wang wrote:
>> netlink kernel socket is protected by refcount, not RCU.
>> Its rcv path is neither protected by RCU. So the synchronize_net()
>> is just pointless.
>
> If I understand correctly, xfrm_user_net_exit() usage of
> RCU_INIT_POINTER() and synchronize_net() is similarly pointless?  Also
> net/phonet/socket.c?  I probably modelled things based on the former...

Possibly, but xfrm case is slightly different, it has two copies of the pointer
to the netlink socket, also it uses exit_batch(). I need to double check.

Take a look at better examples, fib_front, genetlink, rtnetlink.