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@

1 files changed, 2 insertions, 1 deletions

diff --git a/sys/net/if_var.h b/sys/net/if_var.h
index c0bd1ef7459..d98abe02300 100644
--- a/sys/net/if_var.h
+++ b/sys/net/if_var.h
@@ -1,4 +1,4 @@
-/*	$OpenBSD: if_var.h,v 1.66 2015/12/08 10:14:58 dlg Exp $	*/
+/*	$OpenBSD: if_var.h,v 1.67 2015/12/09 03:22:39 dlg Exp $	*/
 /*	$NetBSD: if.h,v 1.23 1996/05/07 02:40:27 thorpej Exp $	*/
 
 /*
@@ -42,6 +42,7 @@
 #include <sys/mbuf.h>
 #include <sys/srp.h>
 #include <sys/refcnt.h>
+#include <sys/task.h>
 #include <sys/time.h>
 
 #include <net/ifq.h>