Re: [PATCH] libstdc++: Make atomic notify_one and notify_all non-const

[Jonathan Wakely <jwakely@redhat.com>]

On Fri, 11 Feb 2022 at 17:40, Thomas Rodgers via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> <recording this here for future reference>
> PR102994 "atomics: std::atomic<ptr>::wait is not marked const" raises the
> issue that the current libstdc++ implementation marks the notify members
> const, the implementation strategy used by libstdc++, as well as libc++
> and the Microsoft STL, do not require the atomic to be mutable (it is hard
> to conceive of a desirable implementation approach that would require it).
> The original paper proposing the wait/notify functionality for atomics
> (p1185) also had these members marked const for the first three revisions,
> but that was changed without explanation in r3 and subsequent revisions of
> the paper.
>
> After raising the issue to the authors of p1185 and the author of the
> libc++ implementation, the consensus seems to be "meh, it's harmless" so
> there seems little appetite for an LWG issue to revisit the subject.
>
> This patch changes the libstdc++ implementation to be in agreement with
> the standard by removing const from those notify_one/notify_all members.
>
> libstdc++-v3/ChangeLog:

Might as well add a "PR libstdc++/102994" here to the bug gets updated
automatically.

OK for trunk with that change.

>         * include/bits/atomic_base.h (atomic_flag::notify_one,
>         notify_all): Remove const qualification.
>         (__atomic_base::notify_one, notify_all): Likewise.
>         * include/std/atomic (atomic<bool>::notify_one, notify_all):
>         Likewise.
>         (atomic::notify_one, notify_all): Likewise.
>         (atomic<T*>::notify_one, notify_all): Likewise.
>         (atomic_notify_one, atomic_notify_all): Likewise.
>         * testsuite/29_atomics/atomic/wait_notify/102994.cc: Adjust test
>         to account for change in notify_one/notify_all signature.
>
> Tested x86_64-pc-linux-gnu.