[committed] libstdc++: Fix build failure in tzdb.cc

[Tomasz Kaminski]

On Wed, Jul 1, 2026 at 5:13 PM Jonathan Wakely <jwakely@redhat.com> wrote:

> On Wed, 1 Jul 2026 at 15:53, Tomasz Kaminski <tkaminsk@redhat.com> wrote:
> >
> >
> >
> > On Wed, Jul 1, 2026 at 4:36 PM Jonathan Wakely <jwakely@redhat.com>
> 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.
>
> Ah yes, so we want:
>
> --- a/libstdc++-v3/src/c++20/tzdb.cc
> +++ b/libstdc++-v3/src/c++20/tzdb.cc
> @@ -1470,9 +1470,15 @@ struct tzdb_list::_Node::NumLeapSeconds
>   void
>   set_locked(unsigned val, const lock_guard<mutex>&)
>   {
> +#if ATOMIC_INT_LOCK_FREE == 2
> +    // Even though the caller locked the mutex, we still need to use an
> +    // atomic store in this case, because there could be concurrent loads.
> +    set(val);
>
Yes, something like this.

> +#else

    // The only caller of this function locks list_mutex() so we would
>     // deadlock if we locked it again here.
>

    count = val;
> +#endif
>   }
>
> private:
>
>
>
>
> >
> >
> >>    }
> >> -#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
> >>
>
>
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