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>

1 files changed, 1 insertions, 1 deletions

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;