On Wed, Jan 31 2024 at  6:19P -0500,
Linus Torvalds [off-list ref] wrote:

On Wed, 31 Jan 2024 at 13:32, Tejun Heo [off-list ref] wrote:

quoted

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.

So the reason I thought it would be a good cleanup to introduce that
"atomic" workqueue thing (now "bh") was that this case literally has a
switch between "use tasklets' or "use workqueues".

So it's not even about "reusing" the workqueue, it's literally a
matter of making it always just use workqueues, and the switch then
becomes just *which* workqueue to use - system or bh.

DM generally always use dedicated workqueues instead of the system.

The dm-crypt tasklet's completion path did punt to the workqueue
otherwise there was use-after-free of the per-bio-data that included
the tasklet. And for verity there was fallback to workqueue if
tasklet-based verification failed. Didn't inspire confidence.

In fact, I suspect there is very little reason ever to *not* just use
the bh one, and even the switch could be removed.

Because I think the only reason the "workqueue of tasklet" choice
existed in the first place was that workqueues were the "proper" data
structure, and the tasklet case was added later as a latency hack, and
everybody knew that tasklets were deprecated.

Correct, abusing tasklets was a very contrived latency optimization.
Happy to see it all go away! (hindsight: it never should have gone in).

Mike