netdev - Re: [PATCH net-next v2 2/5] rhashtable: add function to replace an element

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Message-ID: <CALx6S35sG4+5UT87Yrwa1a=O-Mq09SucXw7=xyxnrJVwXY6fkw@mail.gmail.com>
Date:	Tue, 8 Dec 2015 09:14:24 -0800
From:	Tom Herbert <tom@...bertland.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	David Miller <davem@...emloft.net>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	Kernel Team <kernel-team@...com>
Subject: Re: [PATCH net-next v2 2/5] rhashtable: add function to replace an element

On Tue, Dec 8, 2015 at 1:39 AM, Herbert Xu <herbert@...dor.apana.org.au> wrote:
> David Miller <davem@...emloft.net> wrote:
>> From: Tom Herbert <tom@...bertland.com>
>> Date: Tue, 1 Dec 2015 15:11:09 -0800
>>
>>> +     lock = rht_bucket_lock(tbl, hash);
>>> +
>>> +     spin_lock_bh(lock);
>>> +
>>> +     pprev = &tbl->buckets[hash];
>>> +     rht_for_each(he, tbl, hash) {
>>> +             if (he != obj_old) {
>>> +                     pprev = &he->next;
>>> +                     continue;
>>> +             }
>>> +
>>> +             rcu_assign_pointer(obj_new->next, obj_old->next);
>>> +             rcu_assign_pointer(*pprev, obj_new);
>>> +             err = 0;
>>> +             break;
>>
>> Are you sure this works fine in the presence of both parallel readers and
>> table expansion passes?
>
> Good question.
>
> What's more this is something that can be easily implemented
> outside of rhashtable, i.e., by hashing a pointer to the actual
> object rather than the object itself.  So I'd like to see some
> pretty good reasons for penny-pinching on memory and adding more
> complexity to rhashtable.

That creates one more level of indirection. I don't see how add an
atomic replace operation adds any complexity to the rhashtable, none
of the semantics for rhashtable need to be changed.

>
> 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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