xfs: enable buffer deadlock postmortem diagnosis via ftrace

Create a second buf_trylock tracepoint so that we can distinguish
between a successful and a failed trylock.  With this piece, we can
use a script to look at the ftrace output to detect buffer deadlocks.

[dchinner: update to if/else as per hch's suggestion]
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>

fs/xfs/xfs_buf.c

@@ -944,10 +944,12 @@ xfs_buf_trylock(
 	int			locked;
 
 	locked = down_trylock(&bp->b_sema) == 0;
-	if (locked)
+	if (locked) {
 		XB_SET_OWNER(bp);
-
-	trace_xfs_buf_trylock(bp, _RET_IP_);
+		trace_xfs_buf_trylock(bp, _RET_IP_);
+	} else {
+		trace_xfs_buf_trylock_fail(bp, _RET_IP_);
+	}
 	return locked;
 }
 

fs/xfs/xfs_trace.h

@@ -354,6 +354,7 @@ DEFINE_BUF_EVENT(xfs_buf_submit_wait);
 DEFINE_BUF_EVENT(xfs_buf_bawrite);
 DEFINE_BUF_EVENT(xfs_buf_lock);
 DEFINE_BUF_EVENT(xfs_buf_lock_done);
+DEFINE_BUF_EVENT(xfs_buf_trylock_fail);
 DEFINE_BUF_EVENT(xfs_buf_trylock);
 DEFINE_BUF_EVENT(xfs_buf_unlock);
 DEFINE_BUF_EVENT(xfs_buf_iowait);