[PATCH 7/8] writeback: add lockdep annotation to inode_to_wb()

[Tejun Heo]

With the previous two patches, all operations which acquire wb from
inode are either under one of inode->i_lock, mapping->tree_lock or
wb->list_lock, or protected by stat transaction, which will be
depended upon by foreign inode wb switching.

This patch adds lockdep assertion to inode_to_wb() so that usages
outside the above list locks can be caught easily.
inode_wb_stat_unlocked_begin() is an exception as it's usually
protected by combination of !I_WB_SWITCH and rcu_read_lock().  It's
updated to dereference inode->i_wb directly.

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: Wu Fengguang <fengguang.wu@xxxxxxxxx>
Cc: Greg Thelen <gthelen@xxxxxxxxxx>
---
 include/linux/backing-dev.h | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
index 040be1a..b9937e5 100644
--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -326,10 +326,18 @@ wb_get_create_current(struct backing_dev_info *bdi, gfp_t gfp)
  * inode_to_wb - determine the wb of an inode
  * @inode: inode of interest
  *
- * Returns the wb @inode is currently associated with.
+ * Returns the wb @inode is currently associated with.  The caller must be
+ * holding either @inode->i_lock, @inode->i_mapping->tree_lock, or the
+ * associated wb's list_lock.
  */
 static inline struct bdi_writeback *inode_to_wb(struct inode *inode)
 {
+#ifdef CONFIG_LOCKDEP
+	WARN_ON_ONCE(debug_locks &&
+		     (!lockdep_is_held(&inode->i_lock) &&
+		      !lockdep_is_held(&inode->i_mapping->tree_lock) &&
+		      !lockdep_is_held(&inode->i_wb->list_lock)));
+#endif
 	return inode->i_wb;
 }
 
@@ -360,7 +368,12 @@ inode_wb_stat_unlocked_begin(struct inode *inode, bool *lockedp)
 
 	if (unlikely(*lockedp))
 		spin_lock_irq(&inode->i_mapping->tree_lock);
-	return inode_to_wb(inode);
+
+	/*
+	 * Protected by either !I_WB_SWITCH + rcu_read_lock() or tree_lock.
+	 * inode_to_wb() will bark.  Deref directly.
+	 */
+	return inode->i_wb;
 }
 
 /**
-- 
2.1.0

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