Subject: Re: [PATCH 3/3] Convert the UDP hash lock to RCU

From: Christoph Lameter <[email-masked]>
Date: Tue, 07 Oct 2008 09:15:00 -0500

Eric Dumazet wrote:
>>> Or just add SLAB_DESTROY_BY_RCU to slab creation in proto_register()
>>> for "struct proto udp_prot/udpv6_prot" so that kmem_cache_free() done
>>> in sk_prot_free() can defer freeing to RCU...
>>
>> Be careful!, SLAB_DESTROY_BY_RCU just means the slab page gets
>> RCU-freed, this means that slab object pointers stay pointing to valid
>> memory, but it does _NOT_ mean those slab objects themselves remain
>> valid.
>>
>> The slab allocator is free to re-use those objects at any time -
>> irrespective of the rcu-grace period. Therefore you will have to be able
>> to validate that the object you point to is indeed the object you
>> expect, otherwise strange and wonderful things will happen.
>>
> Thanks for this clarification. I guess we really need a rcu head then :)

No you just need to make sure that the object you located is still active
(f.e. refcount > 0) and that it is really a match (hash pointers may be
updated asynchronously and therefore point to the object that has been reused
for something else).

Generally it is advisable to use SLAB_DESTROY_BY_RCU because it preserves the
cache hot advantages of the objects. Regular RCU freeing will let the object
expire for a tick or so which will result in the cacheline cooling down.