Subject: Re: 3.11-rc6 genetlink locking fix offends lockdep

From: Ding Tianhong <[email-masked]>
Date: Mon, 19 Aug 2013 19:00:03 +0800

On 2013/8/19 16:00, Johannes Berg wrote:
> 
>> 3.11-rc6's commit 58ad436fcf49 ("genetlink: fix family dump race")
>> gives me the lockdep trace below at startup.
> 
> Hmm. Yes, I see now how this happens, not sure why I didn't run into it.
> 
> The problem is that genl_family_rcv_msg() is called with the genl_lock
> held, and then calls netlink_dump_start() with it held, creating a
> genl_lock->cb_mutex dependency, but obviously the dump continuation is
> the other way around.
> 
> We could use the semaphore instead, I believe, but I don't really
> understand the mutex vs. semaphore well enough to be sure that's
> correct.
> 
> johannes
> 
it is useless, the logic need to modify or otherwise it will still call lockdep trace.
maybe i could send a patch for it, if you wish.

> 
> diff --git a/net/netlink/genetlink.c b/net/netlink/genetlink.c
> index f85f8a2..6cfa646 100644
> --- a/net/netlink/genetlink.c
> +++ b/net/netlink/genetlink.c
> @@ -792,7 +792,7 @@ static int ctrl_dumpfamily(struct sk_buff *skb, struct netlink_callback *cb)
>  	bool need_locking = chains_to_skip || fams_to_skip;
>  
>  	if (need_locking)
> -		genl_lock();
> +		down_read(&cb_lock);
>  
>  	for (i = chains_to_skip; i < GENL_FAM_TAB_SIZE; i++) {
>  		n = 0;
> @@ -815,7 +815,7 @@ errout:
>  	cb->args[1] = n;
>  
>  	if (need_locking)
> -		genl_unlock();
> +		up_read(&cb_lock);
>  
>  	return skb->len;
>  }
> 
> 
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>