Re: [PATCH v10 08/14] unwind deferred: Use bitmask to determine which callbacks to call

[Steven Rostedt]

On Fri, 20 Jun 2025 10:15:42 +0200
Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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