netdev - Re: [PATCH 4/6] rhashtable: allow a walk of the hash table without missing objects.

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Message-ID: <20180328060734.GB16291@gondor.apana.org.au>
Date:   Wed, 28 Mar 2018 14:07:34 +0800
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     NeilBrown <neilb@...e.com>
Cc:     Thomas Graf <tgraf@...g.ch>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/6] rhashtable: allow a walk of the hash table without
 missing objects.

On Wed, Mar 28, 2018 at 08:54:41AM +1100, NeilBrown wrote:
>
> Possibly.
> I particularly want the interface to require that you pass the
> previously returned object to _continue. That makes it easy to see that
> the object is still being used.  If someone changes to code to delete
> the object before the _continue, there should be a strong hint that it
> won't work.
> 
> Maybe it would be better to make it a WARN_ON()
> 
>   if (!obj || WARN_ON(iter->p != obj))
>          iter->p = NULL;

This doesn't really protect against the case where obj is removed.
All it proves is that the user saved a copy of obj which we already
did anyway.

To detect an actual removal you'd need to traverse the list.

I have another idea: we could save insert the walkers into the
hash table chain at the end, essentially as a hidden list.  We
can mark it with a flag like rht_marker so that normal traversal
doesn't see it.

That way the removal code can simply traverse that list and inform
them that the iterator is invalid.

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