Re: [PATCH 1/2] rcu: add rcu_access_pointer and rcu_dereference_protect

[Eric Dumazet <eric.dumazet@gmail.com>]

Le mercredi 07 avril 2010 à 17:19 +0100, David Howells a écrit :

> what is the value of sk->sk_wmem_alloc to the lock context of sk->sk_filter?
> Why would lockdep be interested in sk_wmem_alloc?
> 
> Surely, the assertion that the value of sk->sk_filter is related to
> sk_wmem_alloc being 0 is independent of the need to dereference sk_filter for
> RCU purposes.  So why are these being combined?

Because when sk->sk_filter is eventually written by some thread, this
thread _must_ own a reference on the socket, that is sk_wmem_alloc > 0

So when reading sk->sk_filter, the general condition is : 
- We own the rcu lock 
- But on the particular case of __sk_free(),
  we owned the very last reference to sk (we are going to kfree it), so
nobody can possibly change sk->sk_filter under us.