[PATCH v4 7/7] configfs: Fix LOCKDEP nesting issues with fragment semaphores

[Mike Leach]

CoreSight uses configfs to represent the user interface to programmed
configurations, which can be loaded and unloaded dynamically via configfs.

These add and remove configurations using register/unregister group
calls.

It has been found that if CONFIG_LOCKDEP is enabled, then it appears to
be confused by the nesting inherent in the fragment semaphores used
by groups and the underlying subsystem.

This patch sets up a mechanism to use separate classes for the fragment
semaphores, in a similar way to that already in place to fix nesting
issues with the i_mutexes.

Cc: Joel Becker <jlbec at evilplan.org>
Cc: Christoph Hellwig <hch at lst.de>
Signed-off-by: Mike Leach <mike.leach at linaro.org>
---
 fs/configfs/configfs_internal.h |  3 +++
 fs/configfs/dir.c               | 45 +++++++++++++++++++++++++++++++++
 2 files changed, 48 insertions(+)

diff --git a/fs/configfs/configfs_internal.h b/fs/configfs/configfs_internal.h
index c0395363eab9..736c74ec4b7a 100644
--- a/fs/configfs/configfs_internal.h
+++ b/fs/configfs/configfs_internal.h
@@ -22,6 +22,9 @@ struct configfs_fragment {
 	atomic_t frag_count;
 	struct rw_semaphore frag_sem;
 	bool frag_dead;
+#ifdef CONFIG_LOCKDEP
+	int frag_depth;
+#endif
 };
 
 void put_fragment(struct configfs_fragment *);
diff --git a/fs/configfs/dir.c b/fs/configfs/dir.c
index d1f9d2632202..6ecd8961afc3 100644
--- a/fs/configfs/dir.c
+++ b/fs/configfs/dir.c
@@ -133,6 +133,41 @@ configfs_adjust_dir_dirent_depth_after_populate(struct configfs_dirent *sd)
 	sd->s_depth = -1;
 }
 
+/* fragment semaphore needs some lockdep handling */
+static struct lock_class_key default_frag_class[MAX_LOCK_DEPTH];
+
+/*
+ * Set the lockdep depth for a new fragment based on the parent frag depth.
+ * Called from register_subsystem() with NULL parent group to set root subsystem
+ * depth which defaults to 0 in a new fragment, and from register_group() with the
+ * parent group to set a new group fragment based on the parent fragment depth.
+ *
+ * Prevents lockdep getting upset on the unregister_group() call if it cannot
+ * understand the hierarchy of fragments.
+ */
+static void configfs_adjust_frag_depth(struct configfs_fragment *frag,
+				       struct config_group *parent_group)
+{
+	struct configfs_dirent *parent_dirent;
+
+	if (parent_group) {
+		// find parent frag
+		parent_dirent = parent_group->cg_item.ci_dentry->d_fsdata;
+		frag->frag_depth = parent_dirent->s_frag->frag_depth + 1;
+	}
+
+	if (frag->frag_depth < ARRAY_SIZE(default_frag_class)) {
+		lockdep_set_class(&frag->frag_sem,
+				  &default_frag_class[frag->frag_depth]);
+	} else {
+		/*
+		 * In practice the maximum level of locking depth is
+		 * already reached. Just inform about possible reasons.
+		 */
+		pr_info("Too many levels of fragments for the locking correctness validator.\n");
+	}
+}
+
 #else /* CONFIG_LOCKDEP */
 
 static void configfs_init_dirent_depth(struct configfs_dirent *sd)
@@ -154,6 +189,11 @@ configfs_adjust_dir_dirent_depth_after_populate(struct configfs_dirent *sd)
 {
 }
 
+static void configfs_adjust_frag_depth(struct configfs_fragment *frag,
+				       struct config_group *parent_group)
+{
+}
+
 #endif /* CONFIG_LOCKDEP */
 
 static struct configfs_fragment *new_fragment(void)
@@ -165,6 +205,9 @@ static struct configfs_fragment *new_fragment(void)
 		atomic_set(&p->frag_count, 1);
 		init_rwsem(&p->frag_sem);
 		p->frag_dead = false;
+#ifdef CONFIG_LOCKDEP
+		p->frag_depth = 0;
+#endif
 	}
 	return p;
 }
@@ -1742,6 +1785,7 @@ int configfs_register_group(struct config_group *parent_group,
 	parent = parent_group->cg_item.ci_dentry;
 
 	inode_lock_nested(d_inode(parent), I_MUTEX_PARENT);
+	configfs_adjust_frag_depth(frag, parent_group);
 	ret = create_default_group(parent_group, group, frag);
 	if (ret)
 		goto err_out;
@@ -1872,6 +1916,7 @@ int configfs_register_subsystem(struct configfs_subsystem *subsys)
 	mutex_unlock(&configfs_subsystem_mutex);
 
 	inode_lock_nested(d_inode(root), I_MUTEX_PARENT);
+	configfs_adjust_frag_depth(frag, NULL);
 
 	err = -ENOMEM;
 	dentry = d_alloc_name(root, group->cg_item.ci_name);
-- 
2.17.1