afs: Support RCU pathwalk

Make afs_permission() and afs_d_revalidate() do initial checks in RCU-mode
pathwalk to reduce latency in pathwalk elements that get done multiple
times.  We don't need to query the server unless we've received a
notification from it that something has changed or the callback has
expired.

This requires that we can request a key and check permits under RCU
conditions if we need to.

Signed-off-by: David Howells <dhowells@redhat.com>

1 files changed, 53 insertions, 1 deletions

diff --git a/fs/afs/dir.c b/fs/afs/dir.c
index 139b4e3cc946..cc12772d0a4d 100644
--- a/fs/afs/dir.c
+++ b/fs/afs/dir.c
@@ -966,6 +966,58 @@ static struct dentry *afs_lookup(struct inode *dir, struct dentry *dentry,
 }
 
 /*
+ * Check the validity of a dentry under RCU conditions.
+ */
+static int afs_d_revalidate_rcu(struct dentry *dentry)
+{
+	struct afs_vnode *dvnode, *vnode;
+	struct dentry *parent;
+	struct inode *dir, *inode;
+	long dir_version, de_version;
+
+	_enter("%p", dentry);
+
+	/* Check the parent directory is still valid first. */
+	parent = READ_ONCE(dentry->d_parent);
+	dir = d_inode_rcu(parent);
+	if (!dir)
+		return -ECHILD;
+	dvnode = AFS_FS_I(dir);
+	if (test_bit(AFS_VNODE_DELETED, &dvnode->flags))
+		return -ECHILD;
+
+	if (!afs_check_validity(dvnode))
+		return -ECHILD;
+
+	/* We only need to invalidate a dentry if the server's copy changed
+	 * behind our back.  If we made the change, it's no problem.  Note that
+	 * on a 32-bit system, we only have 32 bits in the dentry to store the
+	 * version.
+	 */
+	dir_version = (long)READ_ONCE(dvnode->status.data_version);
+	de_version = (long)READ_ONCE(dentry->d_fsdata);
+	if (de_version != dir_version) {
+		dir_version = (long)READ_ONCE(dvnode->invalid_before);
+		if (de_version - dir_version < 0)
+			return -ECHILD;
+	}
+
+	/* Check to see if the vnode referred to by the dentry still
+	 * has a callback.
+	 */
+	if (d_really_is_positive(dentry)) {
+		inode = d_inode_rcu(dentry);
+		if (inode) {
+			vnode = AFS_FS_I(inode);
+			if (!afs_check_validity(vnode))
+				return -ECHILD;
+		}
+	}
+
+	return 1; /* Still valid */
+}
+
+/*
  * check that a dentry lookup hit has found a valid entry
  * - NOTE! the hit can be a negative hit too, so we can't assume we have an
  *   inode
@@ -982,7 +1034,7 @@ static int afs_d_revalidate(struct dentry *dentry, unsigned int flags)
 	int ret;
 
 	if (flags & LOOKUP_RCU)
-		return -ECHILD;
+		return afs_d_revalidate_rcu(dentry);
 
 	if (d_really_is_positive(dentry)) {
 		vnode = AFS_FS_I(d_inode(dentry));