See upstream commits:
commit
393da91819e35af538ef97c7c6a04899e2fbfe0e
Author: Liu Bo <bo.li.liu@oracle.com>
Date: Fri Jan 5 12:51:16 2018 -0700
Btrfs: add tracepoint for em's EEXIST case
This is adding a tracepoint 'btrfs_handle_em_exist' to help debug the
subtle bugs around merge_extent_mapping.
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>