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

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Message-Id: <1223367480.26330.7.camel@lappy.programming.kicks-ass.net>
Date:	Tue, 07 Oct 2008 10:17:59 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Eric Dumazet <dada1@...mosbay.com>
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

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.

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