On Wed, 1 Jul 2026 at 15:53, Tomasz Kaminski <[email protected]> wrote:
>
>
>
> 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.
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);
+#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
>>
