vfs: improve i_op->atomic_open() documentation

Fix documentation of ->atomic_open() and related functions: finish_open()
and finish_no_open().  Also add details that seem to be unclear and a
source of bugs (some of which are fixed in the following series).

Cc-ing maintainers of all filesystems implementing ->atomic_open().

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Cc: Eric Van Hensbergen <ericvh@gmail.com>
Cc: Sage Weil <sage@inktank.com>
Cc: Steve French <sfrench@samba.org>
Cc: Steven Whitehouse <swhiteho@redhat.com>
Cc: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 18 insertions, 3 deletions

diff --git a/fs/open.c b/fs/open.c
index 2a731b0d08bc..d420331ca32a 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -744,14 +744,24 @@ cleanup_file:
 
 /**
  * finish_open - finish opening a file
- * @od: opaque open data
+ * @file: file pointer
  * @dentry: pointer to dentry
  * @open: open callback
+ * @opened: state of open
  *
  * This can be used to finish opening a file passed to i_op->atomic_open().
  *
  * If the open callback is set to NULL, then the standard f_op->open()
  * filesystem callback is substituted.
+ *
+ * NB: the dentry reference is _not_ consumed.  If, for example, the dentry is
+ * the return value of d_splice_alias(), then the caller needs to perform dput()
+ * on it after finish_open().
+ *
+ * On successful return @file is a fully instantiated open file.  After this, if
+ * an error occurs in ->atomic_open(), it needs to clean up with fput().
+ *
+ * Returns zero on success or -errno if the open failed.
  */
 int finish_open(struct file *file, struct dentry *dentry,
                 int (*open)(struct inode *, struct file *),
@@ -772,11 +782,16 @@ EXPORT_SYMBOL(finish_open);
 /**
  * finish_no_open - finish ->atomic_open() without opening the file
  *
- * @od: opaque open data
+ * @file: file pointer
  * @dentry: dentry or NULL (as returned from ->lookup())
  *
  * This can be used to set the result of a successful lookup in ->atomic_open().
- * The filesystem's atomic_open() method shall return NULL after calling this.
+ *
+ * NB: unlike finish_open() this function does consume the dentry reference and
+ * the caller need not dput() it.
+ *
+ * Returns "1" which must be the return value of ->atomic_open() after having
+ * called this function.
  */
 int finish_no_open(struct file *file, struct dentry *dentry)
 {