On Fri, 20 Jun 2025 10:15:42 +0200
Peter Zijlstra <pet...@infradead.org> wrote:
> On Tue, Jun 10, 2025 at 08:54:29PM -0400, Steven Rostedt wrote:
>
>
> > void unwind_deferred_cancel(struct unwind_work *work)
> > {
> > + struct task_struct *g, *t;
> > +
> > if (!work)
> > return;
> >
> > guard(mutex)(&callback_mutex);
> > list_del(&work->list);
> > +
> > + clear_bit(work->bit, &unwind_mask);
>
> atomic bitop
Yeah, it just seemed cleaner than: unwind_mask &= ~(work->bit);
It's not needed as the update of unwind_mask is done within the
callback_mutex.
>
> > +
> > + guard(rcu)();
> > + /* Clear this bit from all threads */
> > + for_each_process_thread(g, t) {
> > + clear_bit(work->bit, &t->unwind_info.unwind_mask);
> > + }
> > }
> >
> > int unwind_deferred_init(struct unwind_work *work, unwind_callback_t func)
> > @@ -256,6 +278,14 @@ int unwind_deferred_init(struct unwind_work *work,
> > unwind_callback_t func)
> > memset(work, 0, sizeof(*work));
> >
> > guard(mutex)(&callback_mutex);
> > +
> > + /* See if there's a bit in the mask available */
> > + if (unwind_mask == ~0UL)
> > + return -EBUSY;
> > +
> > + work->bit = ffz(unwind_mask);
> > + unwind_mask |= BIT(work->bit);
>
> regular or
>
> > +
> > list_add(&work->list, &callbacks);
> > work->func = func;
> > return 0;
> > @@ -267,6 +297,7 @@ void unwind_task_init(struct task_struct *task)
> >
> > memset(info, 0, sizeof(*info));
> > init_task_work(&info->work, unwind_deferred_task_work);
> > + info->unwind_mask = 0;
> > }
>
> Which is somewhat inconsistent;
>
> __clear_bit()/__set_bit()
Hmm, are the above non-atomic?
>
> or:
>
> unwind_mask &= ~BIT() / unwind_mask |= BIT()
although, because the update is always guarded, this may be the better
approach, as it shows there's no atomic needed.
-- Steve
