svn commit: r220317 - head/sys/net

[Gleb Smirnoff]

Author: glebius
Date: Mon Apr  4 07:45:08 2011
New Revision: 220317
URL: http://svn.freebsd.org/changeset/base/220317

Log:
  When removing ifnets, we should first remove the reference to ifnet
  from the interface index, then decrease refcount, not vice versa.
  
  Otherwise there is a race (reproducible) when if_free_internal()
  contests on IFNET_WLOCK(), and we got a zero-refed ifnet in the
  index for a long time. It may be picked by some other thread,
  that runs ifnet_byindex_ref(), who takes the ifnet from index,
  and bumps refcount. When reader drops the lock, if_free_internal()
  proceeds with free. Then reader tries to free it a second time.

Modified:
  head/sys/net/if.c

Modified: head/sys/net/if.c
==============================================================================
--- head/sys/net/if.c	Mon Apr  4 03:43:13 2011	(r220316)
+++ head/sys/net/if.c	Mon Apr  4 07:45:08 2011	(r220317)
@@ -465,8 +465,8 @@ if_alloc(u_char type)
 }
 
 /*
- * Do the actual work of freeing a struct ifnet, associated index, and layer
- * 2 common structure.  This call is made when the last reference to an
+ * Do the actual work of freeing a struct ifnet, and layer 2 common
+ * structure.  This call is made when the last reference to an
  * interface is released.
  */
 static void
@@ -476,13 +476,6 @@ if_free_internal(struct ifnet *ifp)
 	KASSERT((ifp->if_flags & IFF_DYING),
 	    ("if_free_internal: interface not dying"));
 
-	IFNET_WLOCK();
-	KASSERT(ifp == ifnet_byindex_locked(ifp->if_index),
-	    ("%s: freeing unallocated ifnet", ifp->if_xname));
-
-	ifindex_free_locked(ifp->if_index);
-	IFNET_WUNLOCK();
-
 	if (if_com_free[ifp->if_alloctype] != NULL)
 		if_com_free[ifp->if_alloctype](ifp->if_l2com,
 		    ifp->if_alloctype);
@@ -513,6 +506,14 @@ if_free_type(struct ifnet *ifp, u_char t
 	    ifp->if_alloctype));
 
 	ifp->if_flags |= IFF_DYING;			/* XXX: Locking */
+
+	IFNET_WLOCK();
+	KASSERT(ifp == ifnet_byindex_locked(ifp->if_index),
+	    ("%s: freeing unallocated ifnet", ifp->if_xname));
+
+	ifindex_free_locked(ifp->if_index);
+	IFNET_WUNLOCK();
+
 	if (!refcount_release(&ifp->if_refcount))
 		return;
 	if_free_internal(ifp);