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

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Date:	Tue, 07 Oct 2008 17:07:37 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	paulmck@...ux.vnet.ibm.com, Evgeniy Polyakov <johnpol@....mipt.ru>,
	Corey Minyard <minyard@....org>,
	David Miller <davem@...emloft.net>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	shemminger@...tta.com
Subject: Re: [PATCH 3/3] Convert the UDP hash lock to RCU

Christoph Lameter a écrit :
> Paul E. McKenney wrote:
> 
>> But care is required -- SLAB_DESTROY_BY_RCU permits objects to be freed
>> and reallocated while a reader holds a reference.  The only guarantee is
>> that the -type- of the data structure will not change while a reader holds
>> a reference.  With something like UDP, this might well be sufficient.
> 
> Right so after the hash lookup operation you are not assured that the object
> has not been freed or even reallocated for a different purpose. So after
> finding the pointer to the object two things need to happen (under rcu_lock):
> 
> 1. Verify that the object is still in use
> 2. Verify that the object is matching the hash
> 
> If not then the operation needs to be redone because we have a stale hash pointer.

OK... so restart full lookup at the begining of hash chain if we detect
points 1 or 2 invalid.

Not that expensive since everything should be cache hot :)

One has to take care to group all components (keys to compute the hash, 
and the *next* pointer) in one cache line to minimize cache misses,
since we now need to access them all to compute/check hash value.

Now if a freed object is re-inserted with same hash value, 
same hash chain, we'll also restart the lookup, but it is harmless.




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