On Wed, 31 Jan 2024, Tejun Heo wrote:

Hello,

On Wed, Jan 31, 2024 at 10:19:07PM +0100, Mikulas Patocka wrote:

quoted

quoted

@@ -83,7 +83,7 @@ struct dm_verity_io {
 	struct bvec_iter iter;
 
 	struct work_struct work;
-	struct tasklet_struct tasklet;
+	struct work_struct bh_work;
 
 	/*
 	 * Three variably-size fields follow this struct:

Do we really need two separate work_structs here? They are never submitted 
concurrently, so I think that one would be enough. Or, am I missing 
something?

I don't know, so just did the dumb thing. If the caller always guarantees
that the work items are never queued at the same time, reusing is fine.
However, the followings might be useful to keep on mind:

- work_struct is pretty small - 4 pointers.

- INIT_WORK() on a queued work item isn't gonna be pretty.

- Flushing and no-concurrent-execution guarantee are broken on INIT_WORK().
  e.g. If you queue_work(), INIT_WORK(), flush_work(), the flush isn't
  actually going to wait for the work item to finish. Also, if you do
  queue_work(), INIT_WORK(), queue_work(), the two queued work item
  instances may end up running concurrently.

Muxing a single work item carries more risks of subtle bugs, but in some
cases, the way it's used is clear (e.g. sequential chaining) and that's
fine.

The code doesn't call INIT_WORK() on a queued work item and it doesn't 
flush the workqueue (it destroys it only in a situation when there are no 
work items running) so I think it's safe to use just one work_struct.

Mikulas