linux-kernel - Re: [PATCH - revised] rhashtable: detect when object movement might have invalidated a lookup

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Message-ID: <20180716021653.xtbyetdfc2uzxub3@gondor.apana.org.au>
Date:   Mon, 16 Jul 2018 10:16:53 +0800
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     NeilBrown <neilb@...e.com>
Cc:     David Miller <davem@...emloft.net>, tgraf@...g.ch,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        eric.dumazet@...il.com
Subject: Re: [PATCH - revised] rhashtable: detect when object movement might
 have invalidated a lookup

On Mon, Jul 16, 2018 at 11:23:43AM +1000, NeilBrown wrote:
>
> kmem_cache_free() directly.  For this, I need rhashtable to be safe if
> an object is deleted and immediately re-inserted into the same hash
> chain.

This means that

	rcu_read_lock();
	A = rhashtable_lookup();
	use(A);
	rcu_read_unlock();

A can turn into object B when it is used.  That is just too strange
for words.  Can we see some actual code on how this works?

For comparison, the existing net code where this happens A doesn't
actually change and it simply moves from one hashtable to another.

I'm relucant to add semantics that would restrain on how rhashtable
works unless we have real and valid use-cases for it.

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