Re: [Question] atomic_fetch_andnot() in nohz_idle_balance()

[Peter Zijlstra]

On Wed, Nov 21, 2018 at 11:34:53PM +0100, Andrea Parri wrote:
> Hi,
> 
> The comment for the atomic_fetch_andnot() in nohz_idle_balance() says:
> 
>   "barrier, pairs with nohz_balance_enter_idle(), ensures ..."
> 
> which, well, does sound a note of warning... ;-)
> 
> I see that nohz_balance_enter_idle() has an smp_mb__after_atomic() but
> the comment for the latter suggests that this barrier is pairing with
> the smp_mb() in _nohz_idle_balance().
> 
> So, what is the intended pairing barrier for the atomic_fetch_andnot()?
> what (which memory accesses) do you want "to order" here?

I can't seem to make sense of that comment either; the best I can come
up with is that it would order the prior NOHZ_KICK_MASK load vs us then
changing it.

But that would order against kick_ilb(), not enter_idle.

Vincent?