xfs: only trace buffer items if they exist

The commit 2d3d0c5 ("xfs: lobotomise xfs_trans_read_buf_map()") left
a landmine in the tracing code: trace_xfs_trans_buf_read() is now
call on all buffers that are read through this interface rather than
just buffers in transactions. For buffers outside transaction
context, bp->b_fspriv is null, and so the buf log item tracing
functions cannot be called. This causes a NULL pointer dereference
in the trace_xfs_trans_buf_read() function when tracing is turned
on.

cc: <stable@vger.kernel.org>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

1 files changed, 3 insertions, 2 deletions

diff --git a/fs/xfs/xfs_trans_buf.c b/fs/xfs/xfs_trans_buf.c
index 0a4d4ab6d9a9..75798412859a 100644
--- a/fs/xfs/xfs_trans_buf.c
+++ b/fs/xfs/xfs_trans_buf.c
@@ -327,9 +327,10 @@ xfs_trans_read_buf_map(
                 return -EIO;
         }
 
-        if (tp)
+        if (tp) {
                 _xfs_trans_bjoin(tp, bp, 1);
-        trace_xfs_trans_read_buf(bp->b_fspriv);
+                trace_xfs_trans_read_buf(bp->b_fspriv);
+        }
         *bpp = bp;
         return 0;