[RFC PATCH v3 4/5] libfs: Refactor end-of-directory detection for simple_offset directories

[cel@kernel.org]

From: Chuck Lever <chuck.lever@oracle.com>

This mechanism seems have been misunderstood more than once. Make
the code more self-documentary.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 fs/libfs.c | 54 ++++++++++++++++++++++++++++++++++++++++++------------
 1 file changed, 42 insertions(+), 12 deletions(-)

diff --git a/fs/libfs.c b/fs/libfs.c
index a673427d3416..0deff5390abb 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -449,6 +449,34 @@ void simple_offset_destroy(struct offset_ctx *octx)
 	mtree_destroy(&octx->mt);
 }
 
+static void offset_set_eod(struct file *file)
+{
+	file->private_data = ERR_PTR(-ENOENT);
+}
+
+static void offset_clear_eod(struct file *file)
+{
+	file->private_data = NULL;
+}
+
+static bool offset_at_eod(struct file *file)
+{
+	return file->private_data == ERR_PTR(-ENOENT);
+}
+
+/**
+ * offset_dir_open - Open a directory descriptor
+ * @inode: directory to be opened
+ * @file: struct file to instantiate
+ *
+ * Returns zero on success, or a negative errno value.
+ */
+static int offset_dir_open(struct inode *inode, struct file *file)
+{
+	offset_clear_eod(file);
+	return 0;
+}
+
 /**
  * offset_dir_llseek - Advance the read position of a directory descriptor
  * @file: an open directory whose position is to be updated
@@ -474,8 +502,8 @@ static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence)
 		return -EINVAL;
 	}
 
-	/* In this case, ->private_data is protected by f_pos_lock */
-	file->private_data = NULL;
+	/* ->private_data is protected by f_pos_lock */
+	offset_clear_eod(file);
 	return vfs_setpos(file, offset, LONG_MAX);
 }
 
@@ -506,15 +534,20 @@ static bool offset_dir_emit(struct dir_context *ctx, struct dentry *dentry)
 			  inode->i_ino, fs_umode_to_dtype(inode->i_mode));
 }
 
-static void *offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
+static void offset_iterate_dir(struct file *file, struct dir_context *ctx)
 {
+	struct dentry *dir = file->f_path.dentry;
+	struct inode *inode = d_inode(dir);
 	struct offset_ctx *octx = inode->i_op->get_offset_ctx(inode);
 	struct dentry *dentry;
 
 	while (true) {
 		dentry = offset_find_next(octx, ctx->pos);
-		if (!dentry)
-			return ERR_PTR(-ENOENT);
+		if (!dentry) {
+			/* ->private_data is protected by f_pos_lock */
+			offset_set_eod(file);
+			return;
+		}
 
 		if (!offset_dir_emit(ctx, dentry)) {
 			dput(dentry);
@@ -524,7 +557,6 @@ static void *offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
 		ctx->pos = dentry2offset(dentry) + 1;
 		dput(dentry);
 	}
-	return NULL;
 }
 
 /**
@@ -557,16 +589,14 @@ static int offset_readdir(struct file *file, struct dir_context *ctx)
 	if (!dir_emit_dots(file, ctx))
 		return 0;
 
-	/* In this case, ->private_data is protected by f_pos_lock */
-	if (ctx->pos == DIR_OFFSET_MIN)
-		file->private_data = NULL;
-	else if (file->private_data == ERR_PTR(-ENOENT))
-		return 0;
-	file->private_data = offset_iterate_dir(d_inode(dir), ctx);
+	/* ->private_data is protected by f_pos_lock */
+	if (!offset_at_eod(file))
+		offset_iterate_dir(file, ctx);
 	return 0;
 }
 
 const struct file_operations simple_offset_dir_operations = {
+	.open		= offset_dir_open,
 	.llseek		= offset_dir_llseek,
 	.iterate_shared	= offset_readdir,
 	.read		= generic_read_dir,
-- 
2.47.0