Re: __{read,write}_once

[Maxime Villard <max%m00nbsd.net@localhost>]

Le 11/11/2019 à 13:51, Joerg Sonnenberger a écrit :
> On Mon, Nov 11, 2019 at 11:02:47AM +0100, Maxime Villard wrote:
> > Typically in sys/uvm/uvm_fault.c there are several lockless stat increments
> > like the following:
> >
> > 	/* Increment the counters.*/
> > 	uvmexp.fltanget++;
>
> Wasn't the general consensus here to ideally have per-cpu counters here
> that are aggregated passively?

In this specific case it's not going to work, because uvmexp is supposed to
be read from a core dump, and the stats are expected to be found in the
symbol...

> I can think of three different options depending the platform:
>
> (1) Use full atomic operations. Fine for true UP platforms and when the
> overhead of per-CPU precise accounting is too high.
>
> (2) Use uninterruptible memory operations in per CPU memory, aggregate
> passively on demand.
>
> (3) Emulate uninterruptible memory operations with kernel RAS, aggregate
> passively on demand.

Generally I would do (2), but in this specific case I suggest (1).
atomic_inc_uint is supposed to be implemented on each platform already, so
it's easy to do.

(3) is a big headache for a very small reason, and it's not going to
prevent inter-CPU races.