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Date:      Sun, 22 Sep 2019 20:50:24 +0000 (UTC)
From:      Mateusz Guzik <mjg@FreeBSD.org>
To:        src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org
Subject:   svn commit: r352613 - head/sys/kern
Message-ID:  <201909222050.x8MKoOnM096634@repo.freebsd.org>
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Author: mjg
Date: Sun Sep 22 20:50:24 2019
New Revision: 352613
URL: https://svnweb.freebsd.org/changeset/base/352613

Log:
  cache: try to avoid vhold if locks held
  
  Sponsored by:	The FreeBSD Foundation

Modified:
  head/sys/kern/vfs_cache.c

Modified: head/sys/kern/vfs_cache.c
==============================================================================
--- head/sys/kern/vfs_cache.c	Sun Sep 22 20:49:17 2019	(r352612)
+++ head/sys/kern/vfs_cache.c	Sun Sep 22 20:50:24 2019	(r352613)
@@ -1690,7 +1690,7 @@ cache_enter_time(struct vnode *dvp, struct vnode *vp, 
 	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, 
 		}
 	}
 
+	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, 
 
 	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, 
 out_unlock_free:
 	cache_enter_unlock(&cel);
 	cache_free(ncp);
+	if (held_dvp) {
+		vdrop(dvp);
+		atomic_subtract_long(&numcachehv, 1);
+	}
 	return;
 }

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