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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 -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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