Please review: atomic updates to mbuf's external refcount

[Navdeep Parhar]

I'd like to merge r254341 from user/np/cxl_tuning to head if there are
no objections.  It eliminates a couple of iffy looking constructs in
uipc_mbuf.c

http://svnweb.freebsd.org/base/user/np/cxl_tuning/sys/kern/uipc_mbuf.c?r1=254334&r2=254341&diff_format=u

---------------------
Always increment or decrement an mbuf's external refcount atomically.
Always decrement it in mb_free_ext() so that an external free routine
can safely assert the refcount is 0 (and not 0 or 1) when it's called.
---------------------

Regards,
Navdeep


diff -r 9e9639a7df80 -r 9753d3e51363 sys/kern/uipc_mbuf.c
--- a/sys/kern/uipc_mbuf.c	Thu Aug 29 11:16:04 2013 -0700
+++ b/sys/kern/uipc_mbuf.c	Thu Aug 29 11:16:04 2013 -0700
@@ -282,7 +282,7 @@ m_extadd(struct mbuf *mb, caddr_t buf, u

 /*
  * Non-directly-exported function to clean up after mbufs with M_EXT
- * storage attached to them if the reference count hits 1.
+ * storage attached to them if the reference count hits 0.
  */
 void
 mb_free_ext(struct mbuf *m)
@@ -298,8 +298,7 @@ mb_free_ext(struct mbuf *m)
 	skipmbuf = (m->m_flags & M_NOFREE);

 	/* Free attached storage if this mbuf is the only reference to it. */
-	if (*(m->m_ext.ref_cnt) == 1 ||
-	    atomic_fetchadd_int(m->m_ext.ref_cnt, -1) == 1) {
+	if (atomic_fetchadd_int(m->m_ext.ref_cnt, -1) == 1) {
 		switch (m->m_ext.ext_type) {
 		case EXT_PACKET:	/* The packet zone is special. */
 			if (*(m->m_ext.ref_cnt) == 0)
@@ -367,10 +366,7 @@ mb_dupcl(struct mbuf *n, struct mbuf *m)
 	KASSERT(m->m_ext.ref_cnt != NULL, ("%s: ref_cnt not set", __func__));
 	KASSERT((n->m_flags & M_EXT) == 0, ("%s: M_EXT set", __func__));

-	if (*(m->m_ext.ref_cnt) == 1)
-		*(m->m_ext.ref_cnt) += 1;
-	else
-		atomic_add_int(m->m_ext.ref_cnt, 1);
+	atomic_add_int(m->m_ext.ref_cnt, 1);
 	n->m_ext.ext_buf = m->m_ext.ext_buf;
 	n->m_ext.ext_free = m->m_ext.ext_free;
 	n->m_ext.ext_arg1 = m->m_ext.ext_arg1;