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[PATCH] xfs: only trace buffer items if they exist

To: xfs@xxxxxxxxxxx
Subject: [PATCH] xfs: only trace buffer items if they exist
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Fri, 6 Feb 2015 15:46:48 +1100
Cc: y@xxxxxxxxxxxxxxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
From: Dave Chinner <dchinner@xxxxxxxxxx>

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

Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
 fs/xfs/xfs_trans_buf.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/xfs/xfs_trans_buf.c b/fs/xfs/xfs_trans_buf.c
index 0a4d4ab..7579841 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;

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