Date:   Mon, 10 Dec 2018 09:50:43 +1100
From:   NeilBrown <neilb@...e.com>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Thomas Graf <tgraf@...g.ch>, Tom Herbert <tom@...ntonium.net>,
        David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next] rhashtable: further improve stability of rhashtable_walk

On Fri, Dec 07 2018, Herbert Xu wrote:

> On Wed, Dec 05, 2018 at 02:51:02PM +1100, NeilBrown wrote:
>> 
>> If the sequence:
>>    obj = rhashtable_walk_next(iter);
>>    rhashtable_walk_stop(iter);
>>    rhashtable_remove_fast(ht, &obj->head, params);
>>    rhashtable_walk_start(iter);
>> 
>>  races with another thread inserting or removing
>>  an object on the same hash chain, a subsequent
>>  rhashtable_walk_next() is not guaranteed to get the "next"
>>  object. It is possible that an object could be
>>  repeated, or missed.
>> 
>>  This can be made more reliable by keeping the objects in a hash chain
>>  sorted by memory address.  A subsequent rhashtable_walk_next()
>>  call can reliably find the correct position in the list, and thus
>>  find the 'next' object.
>> 
>>  It is not possible to take this approach with an rhltable as keeping
>>  the hash chain in order is not so easy.  When the first object with a
>>  given key is removed, it is replaced in the chain with the next
>>  object with the same key, and the address of that object may not be
>>  correctly ordered.
>>  I have not yet found any way to achieve the same stability
>>  with rhltables, that doesn't have a major impact on lookup
>>  or insert.  No code currently in Linux would benefit from
>>  such extra stability.
>> 
>>  With this patch:
>>  - a new object is always inserted after the last object with a
>>    smaller address, or at the start.
>>  - when rhashtable_walk_start() is called, it records that 'p' is not
>>    'safe', meaning that it cannot be dereferenced.  The revalidation
>>    that was previously done here is moved to rhashtable_walk_next()
>>  - when rhashtable_walk_next() is called while p is not NULL and not
>>    safe, it walks the chain looking for the first object with an
>>    address greater than p and returns that.  If there is none, it moves
>>    to the next hash chain.
>> 
>> Signed-off-by: NeilBrown <neilb@...e.com>
>> ---
>> 
>> This is a resend of a patch that I sent back in July.  I couldn't
>> applied then because it assumed another rhashtable patch which hadn't
>> landed yet - it now has.
>
> I thought we had agreed to drop this because nobody needs it
> currently and it doesn't handle rhlist?

Hi Herbert,
 I think it was agreed that I would not pursue features that were only
 of use to out-of-tree code, but I don't think that applies here.  This
 is not a feature, this is a quality-of-implementation improvement.
 There are users in the kernel today which use
   rhashtable_walk_stop()/rhashtable_walk_start()
 to drop out of RCU protection for periods during the walk.
 Any such user might miss seeing an object that has been in the table
 for a while - sure that is less than optimal, and should be fixed if
 the cost is small.

 There are currently no rhlist users which use stop/start to drop out
 of RCU, so there is no clear value in fixing that case, or cost in not
 fixing it.

Thanks,
NeilBrown

>
> Cheers,
> -- 
> Email: Herbert Xu <herbert@...dor.apana.org.au>
> Home Page: http://gondor.apana.org.au/~herbert/
> PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

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