[ovs-dev] [PATCH v3] datapath: sample action without side effects

Simon Horman horms at verge.net.au
Thu May 15 00:39:00 UTC 2014

On Wed, May 14, 2014 at 05:22:21PM -0700, Jesse Gross wrote:
> On Wed, May 14, 2014 at 5:05 PM, Simon Horman <horms at verge.net.au> wrote:
> > The sample action is rather generic, allowing arbitrary actions to be
> > executed based on a probability. However its use, within the Open vSwitch
> > code-base is limited: only a single user-space action is ever nested.
> >
> > A consequence of the current implementation of sample actions is that
> > depending on weather the sample action executed (due to its probability)
> > any side-effects of nested actions may or may not be present before
> > executing subsequent actions.  This has the potential to complicate
> > verification of valid actions by the (kernel) datapath. And indeed adding
> > support for push and pop MPLS actions inside sample actions is one case
> > where such case.
> >
> > In order to allow all supported actions to be continue to be nested inside
> > sample actions without the potential need for complex verification code
> > this patch changes the implementation of the sample action in the kernel
> > datapath so that sample actions are more like a function call and any side
> > effects of nested actions are not present when executing subsequent
> > actions.
> >
> > With the above in mind the motivation for this change is twofold:
> >
> > * To contain side-effects the sample action in the hope of making it
> >   easier to deal with in the future and;
> > * To avoid some rather complex verification code introduced in the MPLS
> >   datapath patch.
> >
> > Some notes about the implementation:
> >
> > * This patch silently changes the behaviour of sample actions whose nested
> >   actions have side-effects. There are no known users of such sample
> >   actions.
> >
> > * sample() does not clone the skb for the only known use-case of the sample
> >   action: a single nested userspace action. In such a case a clone is not
> >   needed as the userspace action has no side effects.
> >
> >   Given that there are no known users of other nested actions and in order
> >   to avoid the complexity of predicting if other sequences of actions have
> >   side-effects in such cases the skb is cloned.
> >
> > * As sample() provides a cloned skb in the unlikely case where there are
> >   nested actions other than a single userspace action it is no longer
> >   necessary to clone the skb in do_execute_actions() when executing a
> >   recirculation action just because the keep_skb parameter is set: this
> >   parameter was only set when processing the nested actions of a sample
> >   action.  Moreover it is possible to remove the keep_skb parameter of
> >   do_execute_actions entirely.
> >
> > * As sample() provides either a cloned skb or one that has had a
> >   reference taken (using keep_skb) to do_execute_actions()
> >   the original skb passed to sample() is never consumed. Thus the
> >   caller of sample() (also do_execute_actions()) can use its generic
> >   error handling to free the skb on error.
> This looks good to me but can you provide a signed-off-by line?

Sure, sorry for missing that.
Please let me know if you would like me to repost.

Signed-off-by: Simon Horman <horms at verge.net.au>

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