From 829df1836ff8d758390ad813bd2e24427d85c794 Mon Sep 17 00:00:00 2001
From: kib <kib@FreeBSD.org>
Date: Tue, 15 Sep 2020 22:13:21 +0000
Subject: [PATCH] Microoptimize tmpfs node ref/unref by using atomics.

Avoid tmpfs mount and node locks when ref count is greater than zero,
which is the case until node is being destroyed by unlink or unmount.

Reviewed by:	markj
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
Differential revision:	https://reviews.freebsd.org/D26346
---
 sys/fs/tmpfs/tmpfs.h       |  3 +--
 sys/fs/tmpfs/tmpfs_subr.c  | 35 ++++++++++++++++-------------------
 sys/fs/tmpfs/tmpfs_vnops.c |  2 +-
 3 files changed, 18 insertions(+), 22 deletions(-)

diff --git a/sys/fs/tmpfs/tmpfs.h b/sys/fs/tmpfs/tmpfs.h
index b15a83af7eb..ac8fe0c1c2d 100644
--- a/sys/fs/tmpfs/tmpfs.h
+++ b/sys/fs/tmpfs/tmpfs.h
@@ -228,7 +228,7 @@ struct tmpfs_node {
 	int		tn_vpstate;		/* (i) */
 
 	/* Transient refcounter on this node. */
-	u_int		tn_refcount;		/* (m) + (i) */
+	u_int		tn_refcount;		/* 0<->1 (m) + (i) */
 
 	/* misc data field for different tn_type node */
 	union {
@@ -412,7 +412,6 @@ struct tmpfs_dir_cursor {
  */
 
 void	tmpfs_ref_node(struct tmpfs_node *node);
-void	tmpfs_ref_node_locked(struct tmpfs_node *node);
 int	tmpfs_alloc_node(struct mount *mp, struct tmpfs_mount *, enum vtype,
 	    uid_t uid, gid_t gid, mode_t mode, struct tmpfs_node *,
 	    const char *, dev_t, struct tmpfs_node **);
diff --git a/sys/fs/tmpfs/tmpfs_subr.c b/sys/fs/tmpfs/tmpfs_subr.c
index 7d5d7190014..d9844bc3c7c 100644
--- a/sys/fs/tmpfs/tmpfs_subr.c
+++ b/sys/fs/tmpfs/tmpfs_subr.c
@@ -49,6 +49,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/priv.h>
 #include <sys/proc.h>
 #include <sys/random.h>
+#include <sys/refcount.h>
 #include <sys/rwlock.h>
 #include <sys/stat.h>
 #include <sys/sysctl.h>
@@ -211,21 +212,15 @@ tmpfs_pages_check_avail(struct tmpfs_mount *tmp, size_t req_pages)
 void
 tmpfs_ref_node(struct tmpfs_node *node)
 {
+#ifdef INVARIANTS
+	u_int old;
 
-	TMPFS_NODE_LOCK(node);
-	tmpfs_ref_node_locked(node);
-	TMPFS_NODE_UNLOCK(node);
-}
-
-void
-tmpfs_ref_node_locked(struct tmpfs_node *node)
-{
-
-	TMPFS_NODE_ASSERT_LOCKED(node);
-	KASSERT(node->tn_refcount > 0, ("node %p zero refcount", node));
-	KASSERT(node->tn_refcount < UINT_MAX, ("node %p refcount %u", node,
-	    node->tn_refcount));
-	node->tn_refcount++;
+	old =
+#endif
+	refcount_acquire(&node->tn_refcount);
+#ifdef INVARIANTS
+	KASSERT(old > 0, ("node %p zero refcount", node));
+#endif
 }
 
 /*
@@ -370,6 +365,8 @@ tmpfs_alloc_node(struct mount *mp, struct tmpfs_mount *tmp, enum vtype type,
 void
 tmpfs_free_node(struct tmpfs_mount *tmp, struct tmpfs_node *node)
 {
+	if (refcount_release_if_not_last(&node->tn_refcount))
+		return;
 
 	TMPFS_LOCK(tmp);
 	TMPFS_NODE_LOCK(node);
@@ -384,19 +381,19 @@ tmpfs_free_node_locked(struct tmpfs_mount *tmp, struct tmpfs_node *node,
     bool detach)
 {
 	vm_object_t uobj;
+	bool last;
 
 	TMPFS_MP_ASSERT_LOCKED(tmp);
 	TMPFS_NODE_ASSERT_LOCKED(node);
-	KASSERT(node->tn_refcount > 0, ("node %p refcount zero", node));
 
-	node->tn_refcount--;
-	if (node->tn_attached && (detach || node->tn_refcount == 0)) {
+	last = refcount_release(&node->tn_refcount);
+	if (node->tn_attached && (detach || last)) {
 		MPASS(tmp->tm_nodes_inuse > 0);
 		tmp->tm_nodes_inuse--;
 		LIST_REMOVE(node, tn_entries);
 		node->tn_attached = false;
 	}
-	if (node->tn_refcount > 0)
+	if (!last)
 		return (false);
 
 #ifdef INVARIANTS
@@ -596,7 +593,7 @@ tmpfs_alloc_vp(struct mount *mp, struct tmpfs_node *node, int lkflag,
 	error = 0;
 	tm = VFS_TO_TMPFS(mp);
 	TMPFS_NODE_LOCK(node);
-	tmpfs_ref_node_locked(node);
+	tmpfs_ref_node(node);
 loop:
 	TMPFS_NODE_ASSERT_LOCKED(node);
 	if ((vp = node->tn_vnode) != NULL) {
diff --git a/sys/fs/tmpfs/tmpfs_vnops.c b/sys/fs/tmpfs/tmpfs_vnops.c
index 61dd148c730..c5d0702801e 100644
--- a/sys/fs/tmpfs/tmpfs_vnops.c
+++ b/sys/fs/tmpfs/tmpfs_vnops.c
@@ -1663,7 +1663,7 @@ tmpfs_vptocnp(struct vop_vptocnp_args *ap)
 		if (tnp->tn_type != VDIR)
 			continue;
 		TMPFS_NODE_LOCK(tnp);
-		tmpfs_ref_node_locked(tnp);
+		tmpfs_ref_node(tnp);
 
 		/*
 		 * tn_vnode cannot be instantiated while we hold the
-- 
2.45.0