Re: [RFA] choosing __platform_wait_t on targets without lock-free 64 atomics

[Thomas Rodgers <trodgers@redhat.com>]

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__platform_wait_t should be whatever the platform supports lock free
natively. The use of a 64 bit int there in the fall through was copied from
Olivier's original implementation for libc++, which uses __ulock_wait/wake
on Darwin which takes a unit64_t, because I had intended to add support
Darwin, but haven't done so yet.

On Fri, Dec 30, 2022 at 2:51 AM Iain Sandoe <iain@sandoe.co.uk> wrote:

>
>
> > On 29 Dec 2022, at 17:02, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
> >
> >
> >
> > On Thu, 29 Dec 2022, 16:22 Iain Sandoe, <iain@sandoe.co.uk> wrote:
> >
> >
> > > On 29 Dec 2022, at 15:44, Jonathan Wakely <jwakely.gcc@gmail.com>
> wrote:
> > >
> > >
> > >
> > > On Thu, 29 Dec 2022, 15:30 Iain Sandoe, <iain@sandoe.co.uk> wrote:
> > > Hi Folks,
> > >
> > > > On 29 Dec 2022, at 12:09, Jonathan Wakely <jwakely.gcc@gmail.com>
> wrote:
> > >
> > > > On Thu, 29 Dec 2022, 11:29 Iain Sandoe, <iain@sandoe.co.uk> wrote:
> > >
> > > >> The recent addition of the tz handling has pulled in a dependency
> on </bits/atomic_wait.h>
> > > >>
> > > >> This currently specifies __platform_wait_t as a 64bit quatity on
> platforms without _GLIBCXX_HAVE_LINUX_FUTEX.
> > > >>
> > > >> PowerPC does not have a 64b atomic without library support - so
> that this causes a bootstrap
> > > >> fail on powerpc-darwin (and I guess any other 32b powerpc non-futex
> target).
> > > >>
> > > >> Rather than contrive to build and add libatomic (which is not at
> present available at the point
> > > >> that libstdc++ is built), I wonder if there is any specific reason
> that __platform_wait_t needs
> > > >> to be 64 bits on these platforms? (Especially since the futex case
> uses an int.)
> > > >>
> > > > I think we do want the generic case's _M_wait atomic variable to be
> lock free, otherwise we use two locks for every operation, the one in
> libatomic and the waiter mutex. That's more important than it being any
> specific width.
> > >
> > > Definitely, that’s probably a recipe for some subtle race condition ..
> nested locks etc.
> > >
> > > I didn't see any nested cases from a quick look, but it would still be
> better to avoid two locks.
> > >
> > >
> > > >> Advice on the right way to fix this welcome — as a work-around to
> allow bootstrap to complete
> > > >> I applied the patch below - but that seems unlikely to be the right
> thing generically .
> > > >>
> > > > Rather than __lp64__ I think we should check the
> ATOMIC_LONG_LOCK_FREE macro and use long if it's lock free and int
> otherwise. But Tom needs to confirm that. That would be approximately the
> same as your patch in practice.
> > >
> > > OK.. that makes sense here’s a proposed patch (pending subsequent
> input from Tom).
> > >
> > > I am using “lock free always” as the criterion, “sometimes” does not
> seem useful here.
> > >
> > > Agreed.
> > >
> > >
> > > Although we normally build libstdc++ with the just-built GCC...
> > > .. AFAIK the __SIZEOF_* are available from any version of GCC or clang
> that would
> > > be capable of building the sources.
> > >
> > > Yep, but do we need the size checks at all?
> > >
> > > I was thinking we could just use 'unsigned long' or 'unsigned int'
> directly, instead of a uintN_t typedef. Using the typedef just seems to
> complicate things.
> >
> > That’s fine by me - I was just copying what was there :)
> >
> > In this patch I made it so that a target without a ‘suitable' lock-free
> size would fail to
> > compile the source, which seems better than a link fail later — I could
> make it more
> > specific (e.g. # fail clause) or we could test for smaller lock-free
> entities…
> >
> > I think we can just eschew atomics altogether in that case, and just use
> the mutex for all accesses. I can do that after the break when I'm back
> online.
>
> Great, thanks!
> cheers
> Iain
>
> I’m using this locally in the meantime:
>
> # if  ATOMIC_LONG_LOCK_FREE == 2
>     using __platform_wait_t = long;
> # elif ATOMIC_INT_LOCK_FREE == 2
>     using __platform_wait_t = int;
> # elif ATOMIC_SHORT_LOCK_FREE == 2
>     using __platform_wait_t = short;
> # elif ATOMIC_CHAR_LOCK_FREE == 2
>     using __platform_wait_t = char;
> # else
> # fail No suitable lock-free type found.
> # endif
>
>