lockdep: annotate dir vs file i_mutex

On Mon, 2007-09-24 at 22:13 -0400, Steven Rostedt wrote:
> The circular lock seems to be this:
> 
> #1:
> 
>   sys_mmap2:              down_write(&mm->mmap_sem);
>   nfs_revalidate_mapping: mutex_lock(&inode->i_mutex);
> 
> 
> #0:
> 
>   vfs_readdir:     mutex_lock(&inode->i_mutex);
>    - during the readdir (filldir64), we take a user fault (missing page?)
>     and call do_page_fault -
>   do_page_fault:   down_read(&mm->mmap_sem);
> 
> 
> So it does indeed look like a circular locking. Now the question is, "is
> this a bug?".  Looking like the inode of #1 must be a file or something
> else that you can mmap and the inode of #0 seems it must be a directory.
> I would say "no".
> 
> Now if you can readdir on a file or mmap a directory, then this could be
> an issue.
> 
> Otherwise, I'd love to see someone teach lockdep about this issue! ;-)

Make a distinction between file and dir usage of i_mutex.
The inode should be complete and unused at unlock_new_inode(), re-init
i_mutex depending on its type.
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>

fs/inode.c

@@ -567,6 +567,18 @@ EXPORT_SYMBOL(new_inode);
 
 void unlock_new_inode(struct inode *inode)
 {
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+	struct file_system_type *type = inode->i_sb->s_type;
+	/*
+	 * ensure nobody is actually holding i_mutex
+	 */
+	mutex_destroy(&inode->i_mutex);
+	mutex_init(&inode->i_mutex);
+	if (inode->i_mode & S_IFDIR)
+		lockdep_set_class(&inode->i_mutex, &type->i_mutex_dir_key);
+	else
+		lockdep_set_class(&inode->i_mutex, &type->i_mutex_key);
+#endif
 	/*
 	 * This is special!  We do not need the spinlock
 	 * when clearing I_LOCK, because we're guaranteed

include/linux/fs.h

@@ -1308,6 +1308,7 @@ struct file_system_type {
 
 	struct lock_class_key i_lock_key;
 	struct lock_class_key i_mutex_key;
+	struct lock_class_key i_mutex_dir_key;
 	struct lock_class_key i_alloc_sem_key;
 };