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

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Message-ID: <87in5gt5oa.fsf@notabene.neil.brown.name>
Date:   Mon, 16 Jul 2018 09:55:49 +1000
From:   NeilBrown <neilb@...e.com>
To:     David Miller <davem@...emloft.net>
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

On Wed, Jul 11 2018, David Miller wrote:

> 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

How embarrassing :-(
I had tested a series, but not each individual patch.  Sorry about that.
I'll post the correct patch.

Thanks,
NeilBrown

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