On Wed, Jul 1, 2026 at 4:36 PM Jonathan Wakely <[email protected]> wrote:
> The changes in r17-2047-gb12fdd95251178 cause a bootstrap failure if
> ATOMIC_POINTER_LOCK_FREE != 2 && ATOMIC_INT_LOCK_FREE == 2 is true for
> the target:
>
> .../tzdb.cc: In static member function ‘static const std::chrono::tzdb&
> std::chrono::tzdb_list::_Node::_S_replace_head(std::shared_ptr<std::chrono::tzdb_list::_Node>,
> std::shared_ptr<std::chrono::tzdb_list::_Node>)’:
> .../tzdb.cc:1617:22: error: ‘struct
> std::chrono::tzdb_list::_Node::NumLeapSeconds’ has no member named
> ‘set_locked’
> 1617 |
> num_leap_seconds.set_locked(new_head_ptr->db.leap_seconds.size(), lock);
> | ^~~~~~~~~~
>
> This fix defines the 'set_atomically' and 'set_locked' functions
> unconditionally, and renames the former to just 'set'. This fixes the
> mismatch between the atomic pointer and atomic int conditions, as both
> functions are available for both branches of the #if/#else in
> _Node::_S_replace_head.
>
> libstdc++-v3/ChangeLog:
>
> * src/c++20/tzdb.cc (_Node::NumLeapSeconds::set_atomically):
> Rename to set and define unconditionally.
> (_Node::NumLeapSeconds::set_locked): Define unconditionally.
> ---
>
> Pushed to trunk.
>
> libstdc++-v3/src/c++20/tzdb.cc | 15 +++++++++------
> 1 file changed, 9 insertions(+), 6 deletions(-)
>
> diff --git a/libstdc++-v3/src/c++20/tzdb.cc
> b/libstdc++-v3/src/c++20/tzdb.cc
> index 751370325d76..1aae07033f73 100644
> --- a/libstdc++-v3/src/c++20/tzdb.cc
> +++ b/libstdc++-v3/src/c++20/tzdb.cc
> @@ -1454,23 +1454,26 @@ struct tzdb_list::_Node::NumLeapSeconds
> // Called by _Node::_S_replace_head
> // The two versions are named differently so that caller has to be
> explicit
> // about which version it calls, based on whether the mutex is held.
> -#if ATOMIC_INT_LOCK_FREE == 2
> void
> - set_atomically(unsigned val)
> + set(unsigned val)
> {
> +#if ATOMIC_INT_LOCK_FREE == 2
> atomic_ref<unsigned> ref(count);
> // The release op here synchronizes with the acquire op in get().
> ref.store(val, memory_order::release);
> - }
> #else
> + lock_guard<mutex> l(list_mutex());
> + set_locked(val, l);
> +#endif
> + }
> +
> void
> set_locked(unsigned val, const lock_guard<mutex>&)
> {
> - // XXX The only caller of this function locks list_mutex() so we would
> + // The only caller of this function locks list_mutex() so we would
> // deadlock if we locked it again here.
> count = val;
>
If ATOMIC_INT_LOCK_FREE is true, but USE_ATOMIC_SHARED_PTR is false,
we are having a data race here, as __recent_leap_seconds will not lock the
mutex
before reading count in that case. We should simply call set in this case.
}
> -#endif
>
> private:
> unsigned count = 0;
> @@ -1690,7 +1693,7 @@ constinit tzdb_list::_Node::NumLeapSeconds
> tzdb_list::_Node::num_leap_seconds;
>
> // This allows __recent_leap_second_info() to know that it can use
> // get_tzdb_list()->begin()->leap_seconds to get new leap seconds.
> - num_leap_seconds.set_atomically(new_head_ptr->db.leap_seconds.size());
> + num_leap_seconds.set(new_head_ptr->db.leap_seconds.size());
> #else
> lock_guard<mutex> lock(list_mutex());
> if (const _Node* h = _S_head_owner.get())
> --
> 2.54.0
>
>
