linux-kernel - Re: rhashtable: Kill harmless RCU warning in rhashtable_walk

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Message-ID: <20151218131408.GA21756@gondor.apana.org.au>
Date:	Fri, 18 Dec 2015 21:14:08 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	kernel test robot <ying.huang@...ux.intel.com>, lkp@...org,
	LKML <linux-kernel@...r.kernel.org>,
	Colin Ian King <colin.king@...onical.com>,
	0day robot <fengguang.wu@...el.com>, netdev@...r.kernel.org
Subject: Re: rhashtable: Kill harmless RCU warning in rhashtable_walk_init

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@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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