debugfs: annotate debugfs handlers vs. removal with lockdep

When you take a lock in a debugfs handler but also try
to remove the debugfs file under that lock, things can
deadlock since the removal has to wait for all users
to finish.

Add lockdep annotations in debugfs_file_get()/_put()
to catch such issues.
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

fs/debugfs/file.c

@@ -108,6 +108,12 @@ int debugfs_file_get(struct dentry *dentry)
 			kfree(fsd);
 			fsd = READ_ONCE(dentry->d_fsdata);
 		}
+#ifdef CONFIG_LOCKDEP
+		fsd->lock_name = kasprintf(GFP_KERNEL, "debugfs:%pd", dentry);
+		lockdep_register_key(&fsd->key);
+		lockdep_init_map(&fsd->lockdep_map, fsd->lock_name ?: "debugfs",
+				 &fsd->key, 0);
+#endif
 	}
 
 	/*
@@ -124,6 +130,8 @@ int debugfs_file_get(struct dentry *dentry)
 	if (!refcount_inc_not_zero(&fsd->active_users))
 		return -EIO;
 
+	lock_map_acquire_read(&fsd->lockdep_map);
+
 	return 0;
 }
 EXPORT_SYMBOL_GPL(debugfs_file_get);
@@ -141,6 +149,8 @@ void debugfs_file_put(struct dentry *dentry)
 {
 	struct debugfs_fsdata *fsd = READ_ONCE(dentry->d_fsdata);
 
+	lock_map_release(&fsd->lockdep_map);
+
 	if (refcount_dec_and_test(&fsd->active_users))
 		complete(&fsd->active_users_drained);
 }

fs/debugfs/inode.c

@@ -241,6 +241,14 @@ static void debugfs_release_dentry(struct dentry *dentry)
 	if ((unsigned long)fsd & DEBUGFS_FSDATA_IS_REAL_FOPS_BIT)
 		return;
 
+	/* check it wasn't a dir (no fsdata) or automount (no real_fops) */
+	if (fsd && fsd->real_fops) {
+#ifdef CONFIG_LOCKDEP
+		lockdep_unregister_key(&fsd->key);
+		kfree(fsd->lock_name);
+#endif
+	}
+
 	kfree(fsd);
 }
 
@@ -744,6 +752,10 @@ static void __debugfs_file_removed(struct dentry *dentry)
 	fsd = READ_ONCE(dentry->d_fsdata);
 	if ((unsigned long)fsd & DEBUGFS_FSDATA_IS_REAL_FOPS_BIT)
 		return;
+
+	lock_map_acquire(&fsd->lockdep_map);
+	lock_map_release(&fsd->lockdep_map);
+
 	if (!refcount_dec_and_test(&fsd->active_users))
 		wait_for_completion(&fsd->active_users_drained);
 }

fs/debugfs/internal.h

@@ -7,6 +7,7 @@
 
 #ifndef _DEBUGFS_INTERNAL_H_
 #define _DEBUGFS_INTERNAL_H_
+#include <linux/lockdep.h>
 
 struct file_operations;
 
@@ -23,6 +24,11 @@ struct debugfs_fsdata {
 		struct {
 			refcount_t active_users;
 			struct completion active_users_drained;
+#ifdef CONFIG_LOCKDEP
+			struct lockdep_map lockdep_map;
+			struct lock_class_key key;
+			char *lock_name;
+#endif
 		};
 	};
 };