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

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Message-ID: <48EB7EEA.4040809@linux-foundation.org>
Date:	Tue, 07 Oct 2008 10:23:22 -0500
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Eric Dumazet <dada1@...mosbay.com>
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

Eric Dumazet wrote:

> 
> 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 ?

Because my patches were not accepted that fix the issue.
http://lkml.org/lkml/2006/6/16/144


> 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)

Nothing.

> 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...

If there is a mismatch then you have to do another hash lookup. Do an rcu
unlock and start over.



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