From a2d35545429117e68fbcbc68e14ad55e84265d69 Mon Sep 17 00:00:00 2001
From: Mateusz Guzik <mjg@FreeBSD.org>
Date: Fri, 2 Dec 2016 18:03:15 +0000
Subject: [PATCH] vfs: provide fake locking primitives for the crossmp vnode

Since the vnode is only expected to be shared locked, we can save a
little overhead by only pretending we are locking in the first place.

Reviewed by:	kib
Tested by:	pho
---
 sys/kern/vfs_lookup.c | 63 ++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 59 insertions(+), 4 deletions(-)

diff --git a/sys/kern/vfs_lookup.c b/sys/kern/vfs_lookup.c
index 09f72ef35d18..bab4edf5b444 100644
--- a/sys/kern/vfs_lookup.c
+++ b/sys/kern/vfs_lookup.c
@@ -79,6 +79,64 @@ uma_zone_t namei_zone;
 /* Placeholder vnode for mp traversal. */
 static struct vnode *vp_crossmp;
 
+static int
+crossmp_vop_islocked(struct vop_islocked_args *ap)
+{
+
+	return (LK_SHARED);
+}
+
+static int
+crossmp_vop_lock1(struct vop_lock1_args *ap)
+{
+	struct vnode *vp;
+	struct lock *lk;
+	const char *file;
+	int flags, line;
+
+	vp = ap->a_vp;
+	lk = vp->v_vnlock;
+	flags = ap->a_flags;
+	file = ap->a_file;
+	line = ap->a_line;
+
+	if ((flags & LK_SHARED) == 0)
+		panic("invalid lock request for crossmp");
+
+	WITNESS_CHECKORDER(&lk->lock_object, LOP_NEWORDER, file, line,
+	    flags & LK_INTERLOCK ? &VI_MTX(vp)->lock_object : NULL);
+	WITNESS_LOCK(&lk->lock_object, 0, file, line);
+	if ((flags & LK_INTERLOCK) != 0)
+		VI_UNLOCK(vp);
+	LOCK_LOG_LOCK("SLOCK", &lk->lock_object, 0, 0, ap->a_file, line);
+	return (0);
+}
+
+static int
+crossmp_vop_unlock(struct vop_unlock_args *ap)
+{
+	struct vnode *vp;
+	struct lock *lk;
+	int flags;
+
+	vp = ap->a_vp;
+	lk = vp->v_vnlock;
+	flags = ap->a_flags;
+
+	if ((flags & LK_INTERLOCK) != 0)
+		VI_UNLOCK(vp);
+	WITNESS_UNLOCK(&lk->lock_object, 0, LOCK_FILE, LOCK_LINE);
+	LOCK_LOG_LOCK("SUNLOCK", &lk->lock_object, 0, 0, LOCK_FILE,
+	    LOCK_LINE);
+	return (0);
+}
+
+static struct vop_vector crossmp_vnodeops = {
+	.vop_islocked =		crossmp_vop_islocked,
+	.vop_lock1 =		crossmp_vop_lock1,
+	.vop_unlock =		crossmp_vop_unlock,
+};
+
 struct nameicap_tracker {
 	struct vnode *dp;
 	TAILQ_ENTRY(nameicap_tracker) nm_link;
@@ -95,10 +153,7 @@ nameiinit(void *dummy __unused)
 	    UMA_ALIGN_PTR, 0);
 	nt_zone = uma_zcreate("rentr", sizeof(struct nameicap_tracker),
 	    NULL, NULL, NULL, NULL, sizeof(void *), 0);
-	getnewvnode("crossmp", NULL, &dead_vnodeops, &vp_crossmp);
-	vn_lock(vp_crossmp, LK_EXCLUSIVE);
-	VN_LOCK_ASHARE(vp_crossmp);
-	VOP_UNLOCK(vp_crossmp, 0);
+	getnewvnode("crossmp", NULL, &crossmp_vnodeops, &vp_crossmp);
 }
 SYSINIT(vfs, SI_SUB_VFS, SI_ORDER_SECOND, nameiinit, NULL);