Re: [PATCH v3] rhashtable: detect when object movement between tables might have invalidated a lookup

[David Miller]

From: NeilBrown <neilb@xxxxxxxx>
Date: Fri, 30 Nov 2018 10:26:50 +1100

> 
> Some users of rhashtables might need to move an object from one table
> to another -  this appears to be the reason for the incomplete usage
> of NULLS markers.
> 
> 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
> may not have examined all objects in the target bucket, so it is
> repeated.
> 
> The unique NULLS_MARKER is derived from the address of the
> head of the chain.  As this cannot be derived at load-time the
> static rhnull in rht_bucket_nested() needs to be initialised
> at run time.
> 
> Any caller of a lookup function must still be prepared for the
> possibility that the object returned is in a different table - it
> might have been there for some time.
> 
> Note that this does NOT provide support for other uses of
> NULLS_MARKERs such as allocating with SLAB_TYPESAFE_BY_RCU or changing
> the key of an object and re-inserting it in the same table.
> These could only be done safely if new objects were inserted
> at the *start* of a hash chain, and that is not currently the case.
> 
> Signed-off-by: NeilBrown <neilb@xxxxxxxx>

Applied to net-next.