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

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Message-ID: <20081101031056.GA6955@linux.vnet.ibm.com>
Date:	Fri, 31 Oct 2008 20:10:56 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Eric Dumazet <dada1@...mosbay.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.

On Fri, Oct 31, 2008 at 05:40:46PM +0100, Eric Dumazet wrote:
> 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.

Ah, got it!  Thank you for the explanation!

I was fixating on the low..high interval.  ;-)

							Thanx, Paul
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