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

[Jesse Gross]

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?