Messages in this thread | | | Date | Tue, 7 Oct 2008 07:38:18 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 3/3] Convert the UDP hash lock to RCU |
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On Tue, Oct 07, 2008 at 09:15:00AM -0500, Christoph Lameter wrote: > 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).
In some cases, you might be able to not care, but yes, most of the time, you will need to validate the object.
> 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.
And SLAB_DESTROY_BY_RCU guarantees that the type of the object will remain the same during any given RCU read-side critical section.
Thanx, Paul
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