Re: [PATCHv3 0/2]

[Sean Christopherson <seanjc@xxxxxxxxxx>]

On Fri, Feb 28, 2025, Keith Busch wrote:
> On Fri, Feb 28, 2025 at 06:32:47AM -0800, Sean Christopherson wrote:
> > > diff --git a/include/linux/call_once.h b/include/linux/call_once.h
> > > index ddcfd91493ea..b053f4701c94 100644
> > > --- a/include/linux/call_once.h
> > > +++ b/include/linux/call_once.h
> > > @@ -35,10 +35,12 @@ static inline int call_once(struct once *once, int (*cb)(struct once *))
> > >                 return 0;
> > >  
> > >          guard(mutex)(&once->lock);
> > > -        WARN_ON(atomic_read(&once->state) == ONCE_RUNNING);
> > > -        if (atomic_read(&once->state) != ONCE_NOT_STARTED)
> > > +        if (WARN_ON(atomic_read(&once->state) == ONCE_RUNNING))
> > >                  return -EINVAL;
> > >  
> > > +        if (atomic_read(&once->state) == ONCE_COMPLETED)
> > > +                return 0;
> > > +
> > >          atomic_set(&once->state, ONCE_RUNNING);
> > >         r = cb(once);
> > >         if (r)
> 
> Possible suggestion since it seems odd to do an atomic_read twice on the
> same value.

Yeah, good call.  At the risk of getting too cute, how about this?

static inline int call_once(struct once *once, int (*cb)(struct once *))
{
	int r, state;

	/* Pairs with atomic_set_release() below.  */
	if (atomic_read_acquire(&once->state) == ONCE_COMPLETED)
		return 0;

	guard(mutex)(&once->lock);
	state = atomic_read(&once->state);
	if (unlikely(state != ONCE_NOT_STARTED))
		return WARN_ON_ONCE(state != ONCE_COMPLETED) ? -EINVAL : 0;

	atomic_set(&once->state, ONCE_RUNNING);
	r = cb(once);
	if (r)
		atomic_set(&once->state, ONCE_NOT_STARTED);
	else
		atomic_set_release(&once->state, ONCE_COMPLETED);
	return r;
}