iflib: Drain configuration tasks before interface detach

iflib_device_deregister() sets IFC_IN_DETACH before removing the
interface, but a task which already passed its detach check can still
report a link change.  This can re-arm if_linktask after
ether_ifdetach() has drained it and leave work pending across queue
teardown.

Drain the entire private taskqueue before ether_ifdetach().  Drivers
may register their own link-related configuration tasks there, so
draining only the framework admin task leaves the same race for those
drivers.

MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D58452

Co-authored-by: Andrew Gallatin <gallatin@FreeBSD.org>
Co-authored-by: Kevin Bowling <kbowling@FreeBSD.org>

1 files changed, 7 insertions, 0 deletions

diff --git a/sys/net/iflib.c b/sys/net/iflib.c
index 461ff0ba273e..cda60f174a31 100644
--- a/sys/net/iflib.c
+++ b/sys/net/iflib.c
@@ -5658,6 +5658,13 @@ iflib_device_deregister(if_ctx_t ctx)
 	iflib_unregister_vlan_handlers(ctx);
 
 	iflib_netmap_detach(ifp);
+	/*
+	 * A task that passed its IFC_IN_DETACH check before the flag was set
+	 * can still report a link change.  Drain every private task before
+	 * ether_ifdetach() performs the final if_linktask drain.  Drivers may
+	 * register their own link-related tasks on this taskqueue.
+	 */
+	taskqueue_drain_all(ctx->ifc_tq);
 	ether_ifdetach(ifp);
 
 	CTX_LOCK(ctx);