Lock down global variables in if_gre:

- Add gre_mtx to protect global softc list.
- Hold gre_mtx over various list operations (insert, delete).
- Centralize if_gre interface teardown in gre_destroy(), and call this
  from modevent unload and gre_clone_destroy().
- Export gre_mtx to ip_gre.c, which walks the gre list to look up gre
  interfaces during encapsulation.  Add a wonking comment on how we need
  some sort of drain/reference count mechanism to keep gre references
  alive while in use and simultaneous destroy.

This commit does not lockdown softc data, which follows in a future
commit.

1 files changed, 1 insertions, 0 deletions

diff --git a/sys/net/if_gre.h b/sys/net/if_gre.h
index 21fab39..2910f2f 100644
--- a/sys/net/if_gre.h
+++ b/sys/net/if_gre.h
@@ -176,6 +176,7 @@ struct mobip_h {
 
 #ifdef _KERNEL
 LIST_HEAD(gre_softc_head, gre_softc);
+extern struct mtx gre_mtx;
 extern struct gre_softc_head gre_softc_list;
 
 u_int16_t	gre_in_cksum(u_int16_t *, u_int);