writeback: fix NULL bdi->dev in trace writeback_single_inode

bdi_prune_sb() resets sb->s_bdi to default_backing_dev_info when the
tearing down the original bdi. Fix trace_writeback_single_inode to
use sb->s_bdi=default_backing_dev_info rather than bdi->dev=NULL for a
teared down bdi.

Cc: <stable@kernel.org>
Reported-by: Rabin Vincent <rabin@rab.in>
Tested-by: Rabin Vincent <rabin@rab.in>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>

2 files changed, 9 insertions, 9 deletions

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index f855916657ba..5b4a9362d5aa 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -53,14 +53,6 @@ struct wb_writeback_work {
 };
 
 /*
- * Include the creation of the trace points after defining the
- * wb_writeback_work structure so that the definition remains local to this
- * file.
- */
-#define CREATE_TRACE_POINTS
-#include <trace/events/writeback.h>
-
-/*
  * We don't actually have pdflush, but this one is exported though /proc...
  */
 int nr_pdflush_threads;
@@ -92,6 +84,14 @@ static inline struct inode *wb_inode(struct list_head *head)
         return list_entry(head, struct inode, i_wb_list);
 }
 
+/*
+ * Include the creation of the trace points after defining the
+ * wb_writeback_work structure and inline functions so that the definition
+ * remains local to this file.
+ */
+#define CREATE_TRACE_POINTS
+#include <trace/events/writeback.h>
+
 /* Wakeup flusher thread or forker thread to fork it. Requires bdi->wb_lock. */
 static void bdi_wakeup_flusher(struct backing_dev_info *bdi)
 {
diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
index 8588a8918023..06d302ebcb72 100644
--- a/include/trace/events/writeback.h
+++ b/include/trace/events/writeback.h
@@ -426,7 +426,7 @@ DECLARE_EVENT_CLASS(writeback_single_inode_template,
 
         TP_fast_assign(
                 strncpy(__entry->name,
-                        dev_name(inode->i_mapping->backing_dev_info->dev), 32);
+                        dev_name(inode_to_bdi(inode)->dev), 32);
                 __entry->ino            = inode->i_ino;
                 __entry->state          = inode->i_state;
                 __entry->dirtied_when   = inode->dirtied_when;