rework the if_start mpsafe serialisation so it can serialise arbitrary work

work is represented by struct task.

the start routine is now wrapped by a task which is serialised by the
infrastructure. if_start_barrier has been renamed to ifq_barrier and
is now implemented as a task that gets serialised with the start
routine.

this also adds an ifq_restart() function. it serialises a call to
ifq_clr_oactive and calls the start routine again. it exists to
avoid a race that kettenis@ identified in between when a start
routine discovers theres no space left on a ring, and when it calls
ifq_set_oactive. if the txeof side of the driver empties the ring
and calls ifq_clr_oactive in between the above calls in start, the
queue will be marked oactive and the stack will never call the start
routine again.

by serialising the ifq_set_oactive call in the start routine and
ifq_clr_oactive calls we avoid that race.

tested on various nics
ok mpi@

0 files changed, 0 insertions, 0 deletions