On Tue, Jun 09, 2020 at 05:29:22PM +0000, Luis Chamberlain wrote:

Is scsi-generic is the only unwanted ugly child blktrace has to deal
with? For some reason I thought drivers/md/md.c was one but it seems
like it is not. Do we have an easy way to search for these? I think
this would just affect how we express the comment only.

grep for blk_trace_setup.  For all blk devices that setup comes in
through the block device ioctl path, and that relies on having a
struct block_device and queue.  sg on the other hand calls
blk_trace_setup directly with a NULL bdev argument.

quoted

 		 */
-		dir = q->sg_debugfs_dir;
+		dir = debugfs_create_dir(buts->name, blk_debugfs_root);
+		bt->dir = dir;

The other chicken and egg problem to consider at least in the comments
is that the debugfs directory for these types of devices *have* an
exposed path, but the data structure is rather opaque to the device and
even blktrace.  Fortunately given the recent set of changes around the
q->blk_trace and clarifications around its use we have made it clear now
that so long as hold the q->blk_trace_mutex *and* check q->blk_trace we
*should* not race against two separate creations of debugfs directories,
so I think this is safe, so long as these indpendent drivers don't end
up re-using the same path for some other things later in the future, and
since we have control over what goes under debugfsroot block / I think
we should be good.

But I think that the concern for race on names may still be worth
explaining a bit here.

Feel free to add more comments, but please try to keep them short
and crisp.  At the some point long comments really distract from what
is going on.