netdev - Re: [PATCH resend] rhashtable: detect when object movement might have invalidated a lookup

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Message-Id: <20180711.224658.2077863065492745521.davem@davemloft.net>
Date:   Wed, 11 Jul 2018 22:46:58 -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: 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.