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Date: Tue, 07 Oct 2008 16:50:47 +0200 From: Eric Dumazet <dada1@...mosbay.com> To: Christoph Lameter <cl@...ux-foundation.org> Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>, minyard@....org, Linux Kernel <linux-kernel@...r.kernel.org>, netdev@...r.kernel.org, shemminger@...tta.com, paulmck@...ux.vnet.ibm.com Subject: Re: [PATCH 3/3] Convert the UDP hash lock to RCU Christoph Lameter a écrit : > 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. Seems really good to master this SLAB_DESTROY_BY_RCU thing (I see almost no use of it in current kernel) 1) Hum, do you know why "struct file" objects dont use SLAB_DESTROY_BY_RCU then, since we noticed a performance regression for several workloads at RCUification of file structures ? 2) What prevents an object to be *freed* (and deleted from a hash chain), then re-allocated and inserted to another chain (different keys) ? (final refcount=1) If the lookup detects a key mismatch, how will it continue to the next item, since 'next' pointer will have been reused for the new chain insertion... Me confused... -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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