Re: [PATCH] xen/x86: irq: Avoid a TOCTOU race in pirq_spin_lock_irq_desc()

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Fri, Aug 14, 2020 at 08:25:28PM +0100, Julien Grall wrote:
> Hi Andrew,
> 
> Sorry for the late answer.
> 
> On 23/07/2020 14:59, Andrew Cooper wrote:
> > On 23/07/2020 14:22, Julien Grall wrote:
> > > Hi Jan,
> > > 
> > > On 23/07/2020 12:23, Jan Beulich wrote:
> > > > On 22.07.2020 18:53, Julien Grall wrote:
> > > > > --- a/xen/arch/x86/irq.c
> > > > > +++ b/xen/arch/x86/irq.c
> > > > > @@ -1187,7 +1187,7 @@ struct irq_desc *pirq_spin_lock_irq_desc(
> > > > >          for ( ; ; )
> > > > >        {
> > > > > -        int irq = pirq->arch.irq;
> > > > > +        int irq = read_atomic(&pirq->arch.irq);
> > > > 
> > > > There we go - I'd be fine this way, but I'm pretty sure Andrew
> > > > would want this to be ACCESS_ONCE(). So I guess now is the time
> > > > to settle which one to prefer in new code (or which criteria
> > > > there are to prefer one over the other).
> > > 
> > > I would prefer if we have a single way to force the compiler to do a
> > > single access (read/write).
> > 
> > Unlikely to happen, I'd expect.
> > 
> > But I would really like to get rid of (or at least rename)
> > read_atomic()/write_atomic() specifically because they've got nothing to
> > do with atomic_t's and the set of functionality who's namespace they share.
> 
> Would you be happy if I rename both to READ_ONCE() and WRITE_ONCE()? I would
> also suggest to move them implementation in a new header asm/lib.h.

Maybe {READ/WRITE}_SINGLE (to note those should be implemented using a
single instruction)?

ACCESS_ONCE (which also has the _ONCE suffix) IIRC could be
implemented using several instructions, and hence doesn't seem right
that they all have the _ONCE suffix.

Roger.