Message-ID: <48EB2AE3.3080200@cosmosbay.com>
Date: Tue, 07 Oct 2008 11:24:51 +0200
From: Eric Dumazet <dada1@cosmosbay.com>
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To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: minyard@acm.org, Linux Kernel <linux-kernel@vger.kernel.org>,
       netdev@vger.kernel.org, shemminger@vyatta.com,
       paulmck@linux.vnet.ibm.com
Subject: Re: [PATCH 3/3] Convert the UDP hash lock to RCU
References: <20081006185026.GA10383@minyard.local>	 <48EA8197.6080502@cosmosbay.com> <1223367480.26330.7.camel@lappy.programming.kicks-ass.net>
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Peter Zijlstra a ?crit :
> 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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