Re: [PATCH] event/dsw: fix migration bug

[Jerin Jacob <jerinjacobk@gmail.com>]

On Thu, Jul 7, 2022 at 5:13 PM Mattias Rönnblom <hofors@lysator.liu.se> wrote:
>
> From: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
>
> Fix bug in flow migration, which under certain conditions causes
> reordering and violation of atomicity guarantees.
>
> The issue occurs when the processing of a flow (on an atomic queue)
> has resulted in events enqueued to a flow currently being migrated,
> and the former (producer) flow is also selected for migration. The
> events are buffered ("paused") on the originating port, and released
> (forwarded) when the migration has completed. However, at the time of
> "unpausing" the latter (consumer) flow, processing of the producer
> flow on the port to which it was migrated may have already produced
> events, for the same paused flow. This constitutes a race condition,
> and depending on which port wins, reordering may have been introduced.
>
> This patch forbids migration when a port has paused events, since
> those events may have been the result of processing a to-be-migrated
> flow.
>
> This patch also disallows processing events pertaining to a flow under
> migration, for the same reason. A new buffer is introduced, which
> holds such not-yet-processed events dequeued from the port's input
> ring. Such events are forwarded to the target port as a part of the
> migration process.
>
> The 'forwarding' migration state is eliminated, and instead background
> processing is only performed if there are no unreleased events on the
> port.
>
> The bug is primarily triggered in situations where multiple flows are
> migrated as one transaction, but may occur even if only a single flow
> is migrated (e.g., with older DSW versions, which does not support
> multi-flow migration).
>
> Fixes: f6257b22e767 ("event/dsw: add load balancing")
> Cc: stable@dpdk.org
>
> Signed-off-by: Mattias Rönnblom <mattias.ronnblom@ericsson.com>


+ @Thomas Monjalon  to merge to the main tree directly.