Re: 3.11-rc6 genetlink locking fix offends lockdep

Ding Tianhong
2013-08-19T11:00:03Z

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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> .
>