Re: [PATCH] USB: cdc-acm: fix racy TIOCMIWAIT implementation

[Oliver Neukum]

On 07.09.26 13:38, Johan Hovold wrote:
> On Mon, Sep 07, 2026 at 12:28:46PM +0200, Oliver Neukum wrote:
> > On 07.09.26 12:01, Johan Hovold wrote:
> > > On Mon, Sep 07, 2026 at 11:50:41AM +0200, Oliver Neukum wrote:

> > Yes, but user space can call TIOCGICOUNT multiple times in sequence
> > _without_ calling TIOCMIWAIT in between.
> > In that case the counters have to increase if a line changes, don't they?
> > For that to happen you need to handle a status change in the completion
> > handler.
>
> Yes, the completion handler needs to handle the status change (i.e.
> compare new and previous status and increment the counters), but it
> shouldn't update any old *icount* structure.

We could do that and it would indeed be simpler.

But it would change the semantics so that past status changes
no longer show up in TIOCMIWAIT. Is that really a good idea?

	Regards
		Oliver