netdev - RE: [PATCH net-next 3/3] packet: use percpu mmap tx frame pending refcount

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Date:	Mon, 13 Jan 2014 11:58:30 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Daniel Borkmann' <dborkman@...hat.com>,
	Cong Wang <cwang@...pensource.com>
CC:	David Miller <davem@...emloft.net>, netdev <netdev@...r.kernel.org>
Subject: RE: [PATCH net-next 3/3] packet: use percpu mmap tx frame pending
 refcount

From: Daniel Borkmann
...
> But I think I found a different problem with this idea. It could
> happen with net devices as well, but probably less likely as there
> might be a better distribution of hold/puts among CPUs. However,
> for TX_RING, if we pin the process to a particular CPU, and since
> the destructor is invoked through ksoftirqd, we could end up with
> a misbalance and if the process runs long enough eventually
> overflow for one particular CPU. We could work around that, but I
> think it's not worth the effort.

The sum will be 'correct' when summed across all the cpu even if
one of the values has wrapped - provided all the arithmetic is
unsigned and the variables all the same type.

	David



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