netdev - Re: [PATCH 4/6] rhashtable: allow a walk of the hash table without missing objects.

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Message-ID: <87po3p421q.fsf@notabene.neil.brown.name>
Date:   Wed, 28 Mar 2018 08:54:41 +1100
From:   NeilBrown <neilb@...e.com>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Thomas Graf <tgraf@...g.ch>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/6] rhashtable: allow a walk of the hash table without missing objects.

On Tue, Mar 27 2018, Herbert Xu wrote:

> On Tue, Mar 27, 2018 at 10:33:04AM +1100, NeilBrown wrote:
>>
>> -int rhashtable_walk_start_check(struct rhashtable_iter *iter)
>> +int rhashtable_walk_start_continue(struct rhashtable_iter *iter, struct rhash_head *obj)
>>  	__acquires(RCU)
>>  {
>>  	struct rhashtable *ht = iter->ht;
>>  
>>  	rcu_read_lock();
>>  
>> +	if (!obj || iter->p != obj)
>> +		iter->p = NULL;
>
> Why bother with this check at all? Couldn't we make it so that
> if you call continue then you continue with the cursor otherwise
> you set it to NULL as we currently do.

Possibly.
I particularly want the interface to require that you pass the
previously returned object to _continue. That makes it easy to see that
the object is still being used.  If someone changes to code to delete
the object before the _continue, there should be a strong hint that it
won't work.

Maybe it would be better to make it a WARN_ON()

  if (!obj || WARN_ON(iter->p != obj))
         iter->p = NULL;

??

Thanks,
NeilBrown

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