Luis,
Since commit 80c49563e250 ("scsi: scsi_debug: implement IMMED bit") there are long delays in F_SYNC_DELAY and F_SSU_DELAY. This can cause a memory leak in schedule_resp(), which can be invoked while unloading the scsi_debug module: free_all_queued() had already freed all sd_dp and schedule_resp will alloc a new one, which will never get freed. Here's the kmemleak report while running xfstests generic/350:
Applied to 4.18/scsi-fixes. Thank you!