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

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Date:   Thu, 29 Mar 2018 08:34:45 +1100
From:   NeilBrown <neilb@...e.com>
To:     Herbert Xu <herbert@...dor.apana.org.au>
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, Herbert Xu wrote:

> On Wed, Mar 28, 2018 at 06:17:57PM +1100, NeilBrown wrote:
>>
>> Sounds like over-kill to me.
>> It might be reasonable to have a CONFIG_DEBUG_RHASHTABLE which enables
>> extra to code to catch misuse, but I don't see the justification for
>> always performing these checks.
>> The DEBUG code could just scan the chain (usually quite short) to see if
>> the given element is present.  Of course it might have already been
>> rehashed to the next table, so you would to allow for that possibility -
>> probably check tbl->rehash.
>
> No this is not meant to debug users incorrectly using the cursor.
> This is a replacement of your continue interface by automatically
> validating the cursor.
>
> In fact we can make it even more reliable.  We can insert the walker
> right into the bucket chain, that way the walking will always be
> consistent.
>
> The only problem is that we need be able to differentiate between
> a walker, a normal object, and the end of the list.  I think it
> should be doable.

Yes, I think that could work.  The code to stop over a walker object
during an unlocked search wouldn't be straight forward and would need
careful analysis.

However about storing the hash chains in order by object address?
Then rhashtable_walk_start() can easily find it's place regardless of
whether the old object was still present or not, using <= on the
address.
"Insert" would need to record an insert location and insert there rather
than at the head of the chain.
I might try coding that.

Thanks,
NeilBrown

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