Re: [PATCH] cls_cgroup: read classid atomically in classifier

From: Paul Menage
Date: Tue May 26 2009 - 17:39:57 EST

On Tue, May 26, 2009 at 2:04 PM, Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, 26 May 2009 12:59:11 -0700
> Paul Menage <menage@xxxxxxxxxx> wrote:
>> Avoid reading the unsynchronized value cs->classid multiple times,
>> since it could change concurrently from non-zero to zero; this would
>> result in the classifier returning a positive result with a bogus
>> (zero) classid.
> When this race occurs, what are the user-visible consequences?

My guess is very little - rather than returning -1 to indicate that
there's no classid associated with the flow, we're returning a
positive response that the classid is 0. I don't know the network
scheduling code well enough to predict what effect that would have,
but I'd guess it results in the subsequent classid->queue lookup
failing and the packet being shunted down some default queue (unless
the user happens to have set up a queue with id 0). But the race looks
to be against the intent of the code, which is to return -1 if the
classid is 0, or fill in the res struct if it's non-zero.

> People who need to decide to which kernels a patch should be applied
> like to know such things.

I don't think this is a -stable candidate.

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