Re: __{read,write}_once

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

Le 06/11/2019 à 20:38, Mindaugas Rasiukevicius a écrit :
> Maxime Villard <max%m00nbsd.net@localhost> wrote:
> > There are cases in the kernel where we read/write global memory
> > locklessly, and accept the races either because it is part of the design
> > (eg low-level scheduling) or we simply don't care (eg global stats).
> >
> > In these cases, we want to access the memory only once, and need to ensure
> > the compiler does not split that access in several pieces, some of which
> > may be changed by concurrent accesses. There is a Linux article [1] about
> > this, and also [2]. I'd like to introduce the following macros:
> >
> > <...>
>
> A few comments for everybody here:
>
> - There is a general need in the NetBSD kernel for atomic load/store
> semantics.  This is because plain accesses (loads/stores) are subject
> to _tearing_, _fusing_ and _invented_ loads/stores.  I created a new
> thread which might help to clarify these and various other aspects:
>
> http://mail-index.netbsd.org/tech-kern/2019/11/06/msg025664.html

Thanks

> - In C11, this can be handled with atomic_{load,store}_explicit() and
> memory_order_relaxed (or stronger memory order).
>
> - If we do not want to stick with the C11 API (its emulation), then I
> would suggest to use the similar terminology, e.g. atomic_load_relaxed()
> and atomic_store_relaxed(), or Linux READ_ONCE()/WRITE_ONCE().  There is
> no reason invent new terminology; it might just add more confusion.

But... Linux's READ_ONCE/WRITE_ONCE are not actually atomic, is that correct?
So there is a significant semantic difference.

Also, in my original patch I marked two branches of subr_xcall.c, but it
seems to me now they are actually races: ie xc_wait(), the read of 'xc_donep'
could be made by two 4-byte fetches on 32bit arches (at least). Between the
two, an xc thread could have updated the value. So there is an actual race
which could possibly result in returning early while we shouldn't have. Does
that look correct to you?

Thanks