Re: [PATCH 1/2] rcu: add rcu_access_pointer andrcu_dereference_protect

From: Eric Dumazet
Date: Wed Apr 07 2010 - 12:35:50 EST

Le mercredi 07 avril 2010 Ã 17:19 +0100, David Howells a Ãcrit :

> 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.

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