gfs2: Use GL_NOBLOCK flag for non-blocking lookups

Add the GL_NOBLOCK flag to the locking requests in gfs2_permission() and
gfs2_drevalidate() when called with the MAY_NOT_BLOCK flag and
LOOKUP_RCU flag, respectively.  This will cause the locking requests to
be handled without sleeping if possible.  We bail out with -ECHILD if we
can't grant the glock immediately.

Make sure not to dget() + dput() the parent dentry in gfs2_drevalidate()
in LOOKUP_RCU mode; dput() is a sleeping operation.
Signed-off-by: Abhi Das <adas@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

fs/gfs2/dentry.c

@@ -32,21 +32,25 @@
 
 static int gfs2_drevalidate(struct dentry *dentry, unsigned int flags)
 {
-	struct dentry *parent;
+	struct dentry *parent = NULL;
 	struct gfs2_sbd *sdp;
 	struct gfs2_inode *dip;
-	struct inode *inode;
+	struct inode *dinode, *inode;
 	struct gfs2_holder d_gh;
 	struct gfs2_inode *ip = NULL;
 	int error, valid = 0;
 	int had_lock = 0;
 
-	if (flags & LOOKUP_RCU)
-		return -ECHILD;
-
-	parent = dget_parent(dentry);
-	sdp = GFS2_SB(d_inode(parent));
-	dip = GFS2_I(d_inode(parent));
+	if (flags & LOOKUP_RCU) {
+		dinode = d_inode_rcu(READ_ONCE(dentry->d_parent));
+		if (!dinode)
+			return -ECHILD;
+	} else {
+		parent = dget_parent(dentry);
+		dinode = d_inode(parent);
+	}
+	sdp = GFS2_SB(dinode);
+	dip = GFS2_I(dinode);
 	inode = d_inode(dentry);
 
 	if (inode) {
@@ -62,7 +66,8 @@ static int gfs2_drevalidate(struct dentry *dentry, unsigned int flags)
 
 	had_lock = (gfs2_glock_is_locked_by_me(dip->i_gl) != NULL);
 	if (!had_lock) {
-		error = gfs2_glock_nq_init(dip->i_gl, LM_ST_SHARED, 0, &d_gh);
+		error = gfs2_glock_nq_init(dip->i_gl, LM_ST_SHARED,
+					   flags & LOOKUP_RCU ? GL_NOBLOCK : 0, &d_gh);
 		if (error)
 			goto out;
 	}

fs/gfs2/inode.c

@@ -1882,10 +1882,10 @@ int gfs2_permission(struct mnt_idmap *idmap, struct inode *inode,
 		WARN_ON_ONCE(!may_not_block);
 		return -ECHILD;
         }
-	if (gfs2_glock_is_locked_by_me(gl) == NULL) {
-		if (may_not_block)
-			return -ECHILD;
-		error = gfs2_glock_nq_init(gl, LM_ST_SHARED, LM_FLAG_ANY, &i_gh);
+	if (gfs2_glock_is_locked_by_me(ip->i_gl) == NULL) {
+		int noblock = may_not_block ? GL_NOBLOCK : 0;
+		error = gfs2_glock_nq_init(ip->i_gl, LM_ST_SHARED,
+					   LM_FLAG_ANY | noblock, &i_gh);
 		if (error)
 			return error;
 	}