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

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Message-ID: <20151208093941.GA17795@gondor.apana.org.au>
Date:	Tue, 8 Dec 2015 17:39:41 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	David Miller <davem@...emloft.net>
Cc:	tom@...bertland.com, netdev@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH net-next v2 2/5] rhashtable: add function to replace an
 element

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.

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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