This patch adds the "LOCKSHARED" option to namei which causes it to only acquire shared locks on leafs.

The stat() and open() calls have been changed to make use of this new functionality.  Using shared locks in
these cases is sufficient and can significantly reduce their latency if IO is pending to these vnodes.  Also,
this reduces the number of exclusive locks that are floating around in the system, which helps reduce the
number of deadlocks that occur.

A new kernel option "LOOKUP_SHARED" has been added.  It defaults to off so this patch can be turned on for
testing, and should eventually go away once it is proven to be stable.  I have personally been running this
patch for over a year now, so it is believed to be fully stable.

Reviewed by:	jake, obrien
Approved by:	jake

1 files changed, 63 insertions, 0 deletions

diff --git a/sys/kern/vfs_cache.c b/sys/kern/vfs_cache.c
index 08e0397..529efd1 100644
--- a/sys/kern/vfs_cache.c
+++ b/sys/kern/vfs_cache.c
@@ -570,8 +570,35 @@ vfs_cache_lookup(ap)
 
 	error = cache_lookup(dvp, vpp, cnp);
 
+#ifdef LOOKUP_SHARED
+	if (!error) {
+		/* We do this because the rest of the system now expects to get
+		 * a shared lock, which is later upgraded if LOCKSHARED is not
+		 * set.  We have so many cases here because of bugs that yield
+		 * inconsistant lock states.  This all badly needs to be fixed
+		 */
+		error = VOP_CACHEDLOOKUP(dvp, vpp, cnp);
+		if (!error) {
+			int flock;
+
+			flock = VOP_ISLOCKED(*vpp, td);
+			if (flock != LK_EXCLUSIVE) {
+				if (flock == 0) {
+					if ((flags & ISLASTCN) &&
+					    (flags & LOCKSHARED))
+						VOP_LOCK(*vpp, LK_SHARED, td);
+					else
+						VOP_LOCK(*vpp, LK_EXCLUSIVE, td);
+				}
+			} else if ((flags & ISLASTCN) && (flags & LOCKSHARED))
+				VOP_LOCK(*vpp, LK_DOWNGRADE, td);
+		}
+		return (error);
+	}
+#else
 	if (!error) 
 		return (VOP_CACHEDLOOKUP(dvp, vpp, cnp));
+#endif
 
 	if (error == ENOENT)
 		return (error);
@@ -585,13 +612,28 @@ vfs_cache_lookup(ap)
 	} else if (flags & ISDOTDOT) {
 		VOP_UNLOCK(dvp, 0, td);
 		cnp->cn_flags |= PDIRUNLOCK;
+#ifdef LOOKUP_SHARED
+		if ((flags & ISLASTCN) && (flags & LOCKSHARED))
+			error = vget(vp, LK_SHARED, td);
+		else
+			error = vget(vp, LK_EXCLUSIVE, td);
+#else
 		error = vget(vp, LK_EXCLUSIVE, td);
+#endif
+
 		if (!error && lockparent && (flags & ISLASTCN)) {
 			if ((error = vn_lock(dvp, LK_EXCLUSIVE, td)) == 0)
 				cnp->cn_flags &= ~PDIRUNLOCK;
 		}
 	} else {
+#ifdef LOOKUP_SHARED
+		if ((flags & ISLASTCN) && (flags & LOCKSHARED))
+			error = vget(vp, LK_SHARED, td);
+		else
+			error = vget(vp, LK_EXCLUSIVE, td);
+#else
 		error = vget(vp, LK_EXCLUSIVE, td);
+#endif
 		if (!lockparent || error || !(flags & ISLASTCN)) {
 			VOP_UNLOCK(dvp, 0, td);
 			cnp->cn_flags |= PDIRUNLOCK;
@@ -616,7 +658,28 @@ vfs_cache_lookup(ap)
 			return (error);
 		cnp->cn_flags &= ~PDIRUNLOCK;
 	}
+#ifdef LOOKUP_SHARED
+	error = VOP_CACHEDLOOKUP(dvp, vpp, cnp);
+
+	if (!error) {
+		int flock = 0;
+
+		flock = VOP_ISLOCKED(*vpp, td);
+		if (flock != LK_EXCLUSIVE) {
+			if (flock == 0) {
+				if ((flags & ISLASTCN) && (flags & LOCKSHARED))
+					VOP_LOCK(*vpp, LK_SHARED, td);
+				else
+					VOP_LOCK(*vpp, LK_EXCLUSIVE, td);
+			}
+		} else if ((flags & ISLASTCN) && (flags & LOCKSHARED))
+			VOP_LOCK(*vpp, LK_DOWNGRADE, td);
+	}
+
+	return (error);
+#else
 	return (VOP_CACHEDLOOKUP(dvp, vpp, cnp));
+#endif
 }