From 2187b2a06e2c4352326b9615e46b8efb66e5030d Mon Sep 17 00:00:00 2001
From: kib <kib@FreeBSD.org>
Date: Wed, 10 Jun 2015 02:20:58 +0000
Subject: MFC r283601: Add V_MNTREF flag, to indicate that caller of
 vn_start*_write() already owns a reference on the mount point, and the
 functions can consume it.

---
 sys/kern/vfs_vnops.c | 20 ++++++++++----------
 sys/sys/vnode.h      |  1 +
 2 files changed, 11 insertions(+), 10 deletions(-)

diff --git a/sys/kern/vfs_vnops.c b/sys/kern/vfs_vnops.c
index 1965375..ace8e5c 100644
--- a/sys/kern/vfs_vnops.c
+++ b/sys/kern/vfs_vnops.c
@@ -1619,14 +1619,14 @@ unlock:
 }
 
 int
-vn_start_write(vp, mpp, flags)
-	struct vnode *vp;
-	struct mount **mpp;
-	int flags;
+vn_start_write(struct vnode *vp, struct mount **mpp, int flags)
 {
 	struct mount *mp;
 	int error;
 
+	KASSERT((flags & V_MNTREF) == 0 || (*mpp != NULL && vp == NULL),
+	    ("V_MNTREF requires mp"));
+
 	error = 0;
 	/*
 	 * If a vnode is provided, get and return the mount point that
@@ -1651,7 +1651,7 @@ vn_start_write(vp, mpp, flags)
 	 * emulate a vfs_ref().
 	 */
 	MNT_ILOCK(mp);
-	if (vp == NULL)
+	if (vp == NULL && (flags & V_MNTREF) == 0)
 		MNT_REF(mp);
 
 	return (vn_start_write_locked(mp, flags));
@@ -1665,14 +1665,14 @@ vn_start_write(vp, mpp, flags)
  * time, these operations are halted until the suspension is over.
  */
 int
-vn_start_secondary_write(vp, mpp, flags)
-	struct vnode *vp;
-	struct mount **mpp;
-	int flags;
+vn_start_secondary_write(struct vnode *vp, struct mount **mpp, int flags)
 {
 	struct mount *mp;
 	int error;
 
+	KASSERT((flags & V_MNTREF) == 0 || (*mpp != NULL && vp == NULL),
+	    ("V_MNTREF requires mp"));
+
  retry:
 	if (vp != NULL) {
 		if ((error = VOP_GETWRITEMOUNT(vp, mpp)) != 0) {
@@ -1697,7 +1697,7 @@ vn_start_secondary_write(vp, mpp, flags)
 	 * emulate a vfs_ref().
 	 */
 	MNT_ILOCK(mp);
-	if (vp == NULL)
+	if (vp == NULL && (flags & V_MNTREF) == 0)
 		MNT_REF(mp);
 	if ((mp->mnt_kern_flag & (MNTK_SUSPENDED | MNTK_SUSPEND2)) == 0) {
 		mp->mnt_secondary_writes++;
diff --git a/sys/sys/vnode.h b/sys/sys/vnode.h
index 71b129e..fe4c9ea 100644
--- a/sys/sys/vnode.h
+++ b/sys/sys/vnode.h
@@ -395,6 +395,7 @@ extern int		vttoif_tab[];
 #define	V_WAIT		0x0001	/* vn_start_write: sleep for suspend */
 #define	V_NOWAIT	0x0002	/* vn_start_write: don't sleep for suspend */
 #define	V_XSLEEP	0x0004	/* vn_start_write: just return after sleep */
+#define	V_MNTREF	0x0010	/* vn_start_write: mp is already ref-ed */
 
 #define	VR_START_WRITE	0x0001	/* vfs_write_resume: start write atomically */
 #define	VR_NO_SUSPCLR	0x0002	/* vfs_write_resume: do not clear suspension */
-- 
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