Messages in this thread | | | Date | Tue, 07 Oct 2008 09:15:00 -0500 | From | Christoph Lameter <> | Subject | Re: [PATCH 3/3] Convert the UDP hash lock to RCU |
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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.
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