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

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Date:	Tue, 07 Oct 2008 11:24:51 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	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

Peter Zijlstra a écrit :
> On Mon, 2008-10-06 at 23:22 +0200, 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 :)



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