netdev - Re: [PATCH 2/2] udp: RCU handling for Unicast packets.

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Date:	Thu, 30 Oct 2008 08:04:42 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	David Miller <davem@...emloft.net>
CC:	paulmck@...ux.vnet.ibm.com, minyard@....org, shemminger@...tta.com,
	benny+usenet@...rsen.dk, netdev@...r.kernel.org,
	cl@...ux-foundation.org, a.p.zijlstra@...llo.nl,
	johnpol@....mipt.ru, christian@...i.com
Subject: Re: [PATCH 2/2] udp: RCU handling for Unicast packets.

Eric Dumazet a écrit :
> David Miller a écrit :
>> From: Eric Dumazet <dada1@...mosbay.com>
>> Date: Wed, 29 Oct 2008 23:08:30 +0100
>>
>>> @@ -1746,7 +1753,7 @@ void __init udp_table_init(struct udp_table 
>>> *table)
>>>      int i;
>>>  
>>>      for (i = 0; i < UDP_HTABLE_SIZE; i++) {
>>> -        INIT_HLIST_HEAD(&table->hash[i].head);
>>> +        table->hash[i].head.first = (struct hlist_node *)i;
>>
>> Please hide this behind some list.h interface macro, even something
>> as simple as INIT_HLIST_HEAD_NULLS(X, index) would suffice.
>>
>> And as Corey said, the code needs more comments for something as
>> clever as this! :-)
>>
> 
> Yes I agree 100%, please give me one day to prepare a real patch,
> or else akpm will kill us :)
> 

If we design something that could be reused, say for TCP sockets, we need
to be able to handle very large number of 'NULL' pointers, say, up to 64*1024*1024

So lets use the low order bit, set to one for "NULL" pointers, and 0 for regular pointers.

This gives us 31 bits (or 63 bits) to store any valuable info :)

and all ...._nulls() macros would not need to know the max value (UDP_HTABLE_SIZE in UDP case),
since all they have to do is a test ((unsigned long)pos & 1)

At iterator exit, pos would contain the 'index' value, (pos >> 1), to hide this
implementation detail.


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