On 2021-03-01 09:24, Thiago Macieira via Libstdc++ wrote:
On Sunday, 28 February 2021 07:05:47 PST Hans-Peter Nilsson wrote: On
Fri, 26 Feb 2021, Thiago Macieira via Gcc-patches wrote: ints can be
used in futexes. chars can't.
Shouldn't that be an atomic type instead of a bare int then?
There are a couple of atomic_refs:
using __atomic_phase_ref_t = std::__atomic_ref<__barrier_phase_t>;
using __atomic_phase_const_ref_t = std::__atomic_ref<const
__barrier_phase_t>;
And _M_phase, despite being non-atomic, is never accessed without the
atomic_ref, aside from the constructor. Both arrive() and wait() start
off by
creating the atomic_ref.
If it's non-atomic, then how is wait() supposed to wait on it,
atomically?
But I confess I don't understand this code sufficiently to say it is
correct.
I'm simply saying that waiting on unsigned chars will not use a futex,
at
least until https://lkml.org/lkml/2019/12/4/1373 is merged into the
kernel.
And I am not disagreeing with that. I am, however saying, that I know
this particular implementation (well the upstream one it is based on)
has been extensively tested by the original author (ogiroux) including
time on Summit. If we are going to start changing his design decisions
(beyond the largely cosmetic, not algorithmic, ones that I have made as
per Jonathan's request), they should be motivated by more than a 'well
we feel int's would be better here because Futex' justification.
