Re: git: 8271d9b99a3b - main - libsys: remove usage of pthread_once and _once_stub

On Wed, Feb 21, 2024 at 12:51:04AM +0000, Jessica Clarke wrote:
> On 21 Feb 2024, at 00:29, Konstantin Belousov <kib@FreeBSD.org> wrote:
> > 
> > The branch main has been updated by kib:
> > 
> > URL: https://cgit.FreeBSD.org/src/commit/?id=8271d9b99a3b98c662ee9a6257a144284b7e1728
> > 
> > commit 8271d9b99a3b98c662ee9a6257a144284b7e1728
> > Author:     Konstantin Belousov <kib@FreeBSD.org>
> > AuthorDate: 2024-02-20 14:45:29 +0000
> > Commit:     Konstantin Belousov <kib@FreeBSD.org>
> > CommitDate: 2024-02-21 00:26:11 +0000
> > 
> >    libsys: remove usage of pthread_once and _once_stub
> > 
> >    that existed in auxv.c, use simple bool gate instead. This leaves a
> >    small window if two threads try to call _elf_aux_info(3) simultaneously.
> >    The situation is safe because auxv parsing is really idempotent. The
> >    parsed data is the same, and we store atomic types (int/long/ptr) so
> >    double-init does not matter.
> 
> You still need to load acquire and store release aux_once though,
> otherwise you can see aux_once as true yet read the pre-initialised
> data. In practice that’s surely very hard to hit, but the code as
> written is now wrong. Also, idempotence should probably be made
> unnecessary by using 0/1/2 state for uninitialised/initialising/
> initialised, as it’s still technically UB from a C AM perspective due
> to not being data race free if two threads initialise at the same time.
> Better to just do the correct thing rather than risk things going wrong.

There is too much to handle 'in process' state for loosing thread, I need
the whole libthr machinery.  I added the fences, thanks for noting.

WRT being UB from pure C, we already have much more assumptions about
atomicity.