vfs: improve comment describing fget_light()

Improve the description of fget_light(), which is currently incorrect
about needing a prior refcnt (judging by the way it is actually used).

Signed-off-by: Tony Battersby <tonyb@cybernetics.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/file_table.c

@@ -298,11 +298,20 @@ struct file *fget(unsigned int fd)
 EXPORT_SYMBOL(fget);
 
 /*
- * Lightweight file lookup - no refcnt increment if fd table isn't shared. 
+ * Lightweight file lookup - no refcnt increment if fd table isn't shared.
- * You can use this only if it is guranteed that the current task already 
+ *
- * holds a refcnt to that file. That check has to be done at fget() only
+ * You can use this instead of fget if you satisfy all of the following
- * and a flag is returned to be passed to the corresponding fput_light().
+ * conditions:
- * There must not be a cloning between an fget_light/fput_light pair.
+ * 1) You must call fput_light before exiting the syscall and returning control
+ *    to userspace (i.e. you cannot remember the returned struct file * after
+ *    returning to userspace).
+ * 2) You must not call filp_close on the returned struct file * in between
+ *    calls to fget_light and fput_light.
+ * 3) You must not clone the current task in between the calls to fget_light
+ *    and fput_light.
+ *
+ * The fput_needed flag returned by fget_light should be passed to the
+ * corresponding fput_light.
  */
 struct file *fget_light(unsigned int fd, int *fput_needed)
 {