From 7505cffa56d7347436b32f8d3f192ad1c4383ad8 Mon Sep 17 00:00:00 2001
From: Mateusz Guzik <mjg@FreeBSD.org>
Date: Sun, 22 Sep 2019 20:50:24 +0000
Subject: [PATCH] cache: try to avoid vhold if locks held

Sponsored by:	The FreeBSD Foundation
---
 sys/kern/vfs_cache.c | 30 +++++++++++++++++++++++++++---
 1 file changed, 27 insertions(+), 3 deletions(-)

diff --git a/sys/kern/vfs_cache.c b/sys/kern/vfs_cache.c
index 7b43d4e24d7b..ab55f672a544 100644
--- a/sys/kern/vfs_cache.c
+++ b/sys/kern/vfs_cache.c
@@ -1690,7 +1690,7 @@ cache_enter_time(struct vnode *dvp, struct vnode *vp, struct componentname *cnp,
 	uint32_t hash;
 	int flag;
 	int len;
-	bool neg_locked;
+	bool neg_locked, held_dvp;
 	u_long lnumcache;
 
 	CTR3(KTR_VFS, "cache_enter(%p, %p, %s)", dvp, vp, cnp->cn_nameptr);
@@ -1769,6 +1769,13 @@ cache_enter_time(struct vnode *dvp, struct vnode *vp, struct componentname *cnp,
 		}
 	}
 
+	held_dvp = false;
+	if (LIST_EMPTY(&dvp->v_cache_src) && flag != NCF_ISDOTDOT) {
+		vhold(dvp);
+		atomic_add_long(&numcachehv, 1);
+		held_dvp = true;
+	}
+
 	/*
 	 * Calculate the hash key and setup as much of the new
 	 * namecache entry as possible before acquiring the lock.
@@ -1858,8 +1865,21 @@ cache_enter_time(struct vnode *dvp, struct vnode *vp, struct componentname *cnp,
 
 	if (flag != NCF_ISDOTDOT) {
 		if (LIST_EMPTY(&dvp->v_cache_src)) {
-			vhold(dvp);
-			atomic_add_rel_long(&numcachehv, 1);
+			if (!held_dvp) {
+				vhold(dvp);
+				atomic_add_long(&numcachehv, 1);
+			}
+		} else {
+			if (held_dvp) {
+				/*
+				 * This will not take the interlock as someone
+				 * else already holds the vnode on account of
+				 * the namecache and we hold locks preventing
+				 * this from changing.
+				 */
+				vdrop(dvp);
+				atomic_subtract_long(&numcachehv, 1);
+			}
 		}
 		LIST_INSERT_HEAD(&dvp->v_cache_src, ncp, nc_src);
 	}
@@ -1894,6 +1914,10 @@ cache_enter_time(struct vnode *dvp, struct vnode *vp, struct componentname *cnp,
 out_unlock_free:
 	cache_enter_unlock(&cel);
 	cache_free(ncp);
+	if (held_dvp) {
+		vdrop(dvp);
+		atomic_subtract_long(&numcachehv, 1);
+	}
 	return;
 }