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

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Date:	Fri, 31 Oct 2008 17:40:46 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Corey Minyard <minyard@....org>,
	David Miller <davem@...emloft.net>, shemminger@...tta.com,
	benny+usenet@...rsen.dk, netdev@...r.kernel.org,
	Christoph Lameter <cl@...ux-foundation.org>,
	johnpol@....mipt.ru, Christian Bell <christian@...i.com>
Subject: Re: [PATCH 2/2] udp: RCU handling for Unicast packets.

Paul E. McKenney a écrit :
> On Thu, Oct 30, 2008 at 12:30:20PM +0100, Eric Dumazet wrote:
>> -		while (udp_lib_lport_inuse(net, snum, udptable, sk,
>> -					   saddr_comp)) {
>> +		for (;;) {
>> +			hslot = &udptable->hash[udp_hashfn(net, snum)];
>> +			spin_lock_bh(&hslot->lock);
>> +			if (!udp_lib_lport_inuse(net, snum, hslot, sk, saddr_comp))
>> +				break;
>> +			spin_unlock_bh(&hslot->lock);
>>  			do {
>>  				snum = snum + rand;
>>  			} while (snum < low || snum > high);
> 
> The above -really- confuses me, but not part of this patch.  If we are
> out of range, keep going?  Well, I guess that since it is a short, we
> cannot go very far...
> 
>>  			if (snum == first)
>>  				goto fail;
> 
> And I don't understand how we are guaranteed to have scanned all the
> possible ports upon failure, but happy to leave that to you guys.

Well, we have 65536(=2^16) possible port values, and while 'rand' is random,
it has the interesting property/bias of being odd.

We know (thanks modular arithmetic / congruence relation) we will hit
all 65356 values exactly once, after exactly 65536 iterations.

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