I was going through my set of kernel patches that I've cherry picked on
LKML for my private kernel, and I noticed this hasn't gotten merged into
mainline yet.   The original thread was here:

	 http://lkml.org/lkml/2007/10/27/61

and addressed a panic after a umount reported by Sebastian Siewior.  Was
this addressed in some other way?

          					- Ted


diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index 61fdaf0..4042269 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -1115,6 +1115,8 @@ static int scsi_init_io(struct scsi_cmnd *cmd)
 	 * kmapping pages)
 	 */
 	cmd->use_sg = req->nr_phys_segments;
+	if (!cmd->use_sg)
+		return 0;
 
 	/*
 	 * If sg table allocation fails, requeue request later.
@@ -1191,7 +1193,7 @@ int scsi_setup_blk_pc_cmnd(struct scsi_device *sdev, struct request *req)
 	if (req->bio) {
 		int ret;
 
-		BUG_ON(!req->nr_phys_segments);
+		BUG_ON(!req->nr_phys_segments && req->bio->bi_size);
 
 		ret = scsi_init_io(cmd);
 		if (unlikely(ret))
@@ -1236,9 +1238,11 @@ int scsi_setup_fs_cmnd(struct scsi_device *sdev, struct request *req)
 	if (ret != BLKPREP_OK)
 		return ret;
 	/*
-	 * Filesystem requests must transfer data.
+	 * Filesystem requests must transfer data, unless it's an empty
+	 * barrier.
 	 */
-	BUG_ON(!req->nr_phys_segments);
+	if (!req->nr_phys_segments && !bio_empty_barrier(req->bio))
+		blk_dump_rq_flags(req, "scsi");
 
 	cmd = scsi_get_cmd_from_req(sdev, req);
 	if (unlikely(!cmd))

--
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