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Date:   Wed, 11 Jul 2018 22:48:01 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     neilb@...e.com
Cc:     herbert@...dor.apana.org.au, tgraf@...g.ch, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, eric.dumazet@...il.com
Subject: Re: [PATCH resend] rhashtable: detect when object movement might
 have invalidated a lookup

From: David Miller <davem@...emloft.net>
Date: Wed, 11 Jul 2018 22:46:58 -0700 (PDT)

> From: NeilBrown <neilb@...e.com>
> Date: Fri, 06 Jul 2018 17:08:35 +1000
> 
>> 
>> 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.
>> 
>> 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
>> that it was not re-added *after* its original location, or a lookup may
>> incorrectly fail.  The easiest solution is to ensure it is inserted at
>> the start of the chain.  insert_slow() already does that,
>> insert_fast() does not.  So this patch changes insert_fast to always
>> insert at the head of the chain.
>> 
>> Note that such a user must do their own double-checking of
>> the object found by rhashtable_lookup_fast() after ensuring
>> mutual exclusion which anything that might change the key, such as
>> successfully taking a new reference.
>> 
>> Signed-off-by: NeilBrown <neilb@...e.com>
> 
> Applied to net-next.

Actually, reverted, it doesn't even compile.

lib/rhashtable.c: In function ‘rht_bucket_nested’:
lib/rhashtable.c:1187:39: error: macro "INIT_RHT_NULLS_HEAD" passed 3 arguments, but takes just 1
    INIT_RHT_NULLS_HEAD(rhnull, NULL, 0);
                                       ^
lib/rhashtable.c:1187:4: error: ‘INIT_RHT_NULLS_HEAD’ undeclared (first use in this function); did you mean ‘INIT_LIST_HEAD’?
    INIT_RHT_NULLS_HEAD(rhnull, NULL, 0);
    ^~~~~~~~~~~~~~~~~~~
    INIT_LIST_HEAD
lib/rhashtable.c:1187:4: note: each undeclared identifier is reported only once for each function it appears in

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