linux-kernel - Re: [PATCH 04/18] rhashtable: detect when object movement might have invalidated a lookup

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Date:   Mon, 04 Jun 2018 13:38:46 +1000
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,
        Eric Dumazet <eric.dumazet@...il.com>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH 04/18] rhashtable: detect when object movement might have invalidated a lookup

On Sat, Jun 02 2018, Herbert Xu wrote:

> On Fri, Jun 01, 2018 at 02:44:09PM +1000, NeilBrown wrote:
>> Some users of rhashtable might need to change the key
>> of an object and move it to a different location in the table.
>> Other users might want to allocate objects using
>> SLAB_TYPESAFE_BY_RCU which can result in the same memory allocation
>> being used for a different (type-compatible) purpose and similarly
>> end up in a different hash-chain.
>>
>> To support these, we store a unique NULLS_MARKER at the end of
>> each chain, and when a search fails to find a match, we check
>> if the NULLS marker found was the expected one.  If not,
>> the search is repeated.
>> 
>> The unique NULLS_MARKER is derived from the address of the
>> head of the chain.
>
> Yes I thinks makes a lot more sense than the existing rhashtable
> nulls code.  The current rhashtable nulls code harkens back to the
> time of the old rhashtable implementation where the same chain
> existed in two different tables and that is no longer the case.
>
>> If an object is removed and re-added to the same hash chain, we won't
>> notice by looking that the NULLS marker.  In this case we must be sure
>
> This is not currently required by TCP/UDP.  I'd rather not add
> extra constraints that aren't actually used.

I assume you are referring here to the change to insert at the head of
the chain, rather than at the end.
This is important for me to be able to use SLAB_TYPESAFE_BY_RCU
for the kmem_cache.  Without that, I need a new rcu-free callback
for every different kmem_cache.
When using SLAB_TYPESAFE_BY_RCU, there is no way to know where
a freed and re-allocated object might be used again - it could
be used in the same table with the same hash value.

Thanks,
NeilBrown

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