Re: [patch 3/3] tick: Annotate tick_do_timer_cpu data races

[Paul E. McKenney]

On Thu, Dec 17, 2020 at 11:48:23AM +0100, Peter Zijlstra wrote:
> On Wed, Dec 16, 2020 at 01:19:31PM -0800, Paul E. McKenney wrote:
> > Given that there is no optimization potential, then the main reason to use
> > data_race() instead of *_ONCE() is to prevent KCSAN from considering the
> > accesses when looking for data races.  But that is mostly for debugging
> > accesses, in cases when these accesses are not really part of the
> > concurrent algorithm.
> > 
> > So if I understand the situation correctly, I would be using *ONCE().
> 
> Huh, what, why?
> 
> The code doesn't need READ_ONCE(), it merely wants to tell kasan that
> the race it observes is fine and as to please shut up.
> 
> IOW data_race() is accurate and right.

Other way around.

The code does not need any of the compiler optimizations that might
someday be enabled by data_race().  So why do you need data_race() here?
What do exactly is lost by instead using READ_ONCE()?

							Thanx, Paul