Here's why:

This stage is needed to make sure all tasks have appeared
and did some actions (that are called before restore_finish_stage()).
With this description there's no need in involving criu process
in it, this stage is purely inter-tasks sync point.

Taking into account we do already make root task wait for others
to complete forking (it calls restore_wait_ther_tasks()) we may
rework this stage not to involve criu process in it.

Here's how:

So the criu task starts the forking stage, then goes waiting for
"inprogress tasks". The latter wait is purely about nr_in_progress
counter, thus there's no strict requirement that the stage remains
the same by the time criu is woken up.

Siad that, the root task waits for other tasks to finish forking,
does fini_restore_mntns() (already in the code), then switches
the stage to the next (the RESTORE one). Other tasks do normal
staging barrier. Criu task is not woken up as nr_in_progress
always remains >= 1.

The result is -2 context switches -- from root task to criu and
back -- which gives us good boost when restoring single task app.
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>