Re: rhashtable: Kill harmless RCU warning in rhashtable_walk_init

[Herbert Xu]

On Fri, Dec 18, 2015 at 04:54:14AM -0800, Eric Dumazet wrote:
>
> You can avoid the comment by using the self documented and lockdep
> enabled primitive
> 
> iter->walker->tbl = rcu_dereference_protected(ht->tbl,
> 					      lockdep_is_held(&ht->lock));

That is just gross.  I think a comment is much better in this case.

If we were to have more place where ht->lock is taken and we had
to do the RCU dereference on ht->tbl then we could add a helper
for it.  For now it's just a single place and I think a comment
is the best way to deal with it.

> But, storing the ht->tbl and then releasing the lock immediately after
> escapes RCU protection.
> 
> So why do we store ht->tbl in the first place ?
> 
> What exactly prevents it from disappearing after lock is released ?

We add ourselves to the walker list before we release the lock.
The only entity that can destroy ht->tbl will take care of all
walkers before doing so.

Cheers,
-- 
Email: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Home Page: http://gondor.apana.org.au/~herbert/
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