[PATCH] libstdc++: Remove dependency on std::atomic<unsigned> in tzdb.cc

[Tomasz Kaminski]

On Thu, Jun 25, 2026 at 2:17 PM Jonathan Wakely <jwakely@redhat.com> wrote:

> On Thu, 25 Jun 2026 at 13:08, Jonathan Wakely <jwakely@redhat.com> wrote:
> >
> > On Thu, 25 Jun 2026 at 13:01, Tomasz Kaminski <tkaminsk@redhat.com>
> wrote:
> > >
> > >
> > >
> > > On Thu, Jun 25, 2026 at 1:42 PM Jonathan Wakely <jwakely@redhat.com>
> wrote:
> > >>
> > >> My r17-471-ge79f0f818c0e42 change to optimize handling of leap seconds
> > >> introduced a hard dependency on std::atomic<unsigned>, which causes
> > >> problems for targets without atomic word operations, like Cortex-M0.:
> > >> https://gcc.gnu.org/pipermail/gcc-patches/2026-June/719704.html
> > >>
> > >> This patch replaces the num_leap_seconds variable with a struct which
> > >> decides whether to use std::atomic_ref<unsigned> or perform all
> accesses
> > >> while holding a lock on the pre-existing mutex used for the tzdb_list
> > >> singleton.
> > >>
> > >> The workaround is a bit ugly and fragile, because it assumes that
> there
> > >> is only one caller of num_leap_seconds.set and that the list_mutex()
> is
> > >> locked by that caller iff the tzdb_list doesn't use
> atomic<shared_ptr<>>
> > >> (which is enforced via preprocessor checks).
> > >
> > > We could have two different methods, set_under_lock and set_atomic
> > > and call them in branches:
> >
> > I considered that, but it means duplicating the _S_cache_list_head
> > call, because ...
> >
> >
> > > #if USE_ATOMIC_SHARED_PTR
> > >     new_head_ptr->next = curr;
> > >     while (!_S_head_owner.compare_exchange_strong(curr, new_head))
> > >       {
> > >         if (curr->db.version == new_head_ptr->db.version)
> > >           return curr->db;
> > >         new_head_ptr->next = curr;
> > >       }
> > >     // XXX small window here where _S_head_cache still points to
> previous tzdb.
> > >      _S_cache_list_head(new_head_ptr); <-- This is empty if
> USE_ATOMIC_SHARED_PTR is false,
> > >
> so do not define it.
> >
> > Strictly speaking, it's empty for ! USE_ATOMIC_LIST_HEAD and in theory
> > the code is designed to support the case where we have
> > USE_ATOMIC_LIST_HEAD and ! USE_ATOMIC_SHARED_PTR. For example, if
> > benchmarking shows that the mutex performs better than the
> > atomic<shared_ptr<>>, maybe only on particular platforms.
> >
> > So it's true that _S_cache_list_head is empty for !
> > USE_ATOMIC_SHARED_PTR *today* but the design doesn't require that.
> >
> >
> >
> > >     set_lockfree(....)
> > > #else
> > >     lock_guard<mutex> l(list_mutex());
> > >     if (const _Node* h = _S_head_owner.get())
> > >       {
> > >         if (h->db.version == new_head_ptr->db.version)
> > >           return h->db;
> > >         new_head_ptr->next = _S_head_owner;
> > >       }
> > >     _S_head_owner = std::move(new_head);
> > >     set_atomic(....)
> > > #endif
> > > I think I would preffer that.
>
>
> How about doing this instead:
>
I am not convinced that if we ever add a non-empty _S_head_cache, it will
not be moved
inside block. If we use mutex (instead of atomic<shared_ptr>) we will need
to deal with the
the fact that a different mutex is already taken, thus avoiding deadlock.

The suggested change has exactly the same number of lines, as duplicating
_S_head_cache
would cause, and it simply "smarter" (not in positive way) than alternative
of using different names.


@@ -1583,6 +1583,7 @@ constinit tzdb_list::_Node::NumLeapSeconds
> tzdb_list::_Node::num_leap_seconds;
>   {
>     _Node* new_head_ptr = new_head.get();
> #if USE_ATOMIC_SHARED_PTR
> +    const int lock = 0; // dummy variable to pass to num_leap_seconds.set.
>     new_head_ptr->next = curr;
>     while (!_S_head_owner.compare_exchange_strong(curr, new_head))
>       {
> @@ -1592,7 +1593,7 @@ constinit tzdb_list::_Node::NumLeapSeconds
> tzdb_list::_Node::num_leap_seconds;
>       }
>     // XXX small window here where _S_head_cache still points to previous
> tzdb.
> #else
> -    lock_guard<mutex> l(list_mutex());
> +    lock_guard<mutex> lock(list_mutex());
>     if (const _Node* h = _S_head_owner.get())
>       {
>        if (h->db.version == new_head_ptr->db.version)
> @@ -1605,7 +1606,7 @@ 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(new_head_ptr->db.leap_seconds.size());
> +    num_leap_seconds.set(new_head_ptr->db.leap_seconds.size(), lock);
>
>     return new_head_ptr->db;
>   }
>
>
> And then define the set member like this:
>
>   // Called by _Node::_S_replace_head
> #if ATOMIC_INT_LOCK_FREE == 2
>   void
>   set(unsigned val, int)
>   {
>     atomic_ref<unsigned> ref(count);
>     // The release op here synchronizes with the acquire op in get().
>     ref.store(val, memory_order::release);
>   }
> #else
>   void
>   set(unsigned val, const lock_guard<mutex>&)
>   {
>     // XXX The only caller of this function locks list_mutex() so we would
>     // deadlock if we locked it again here.
>     count = val;
>   }
> #endif
>
>
> If something ever gets out of sync and we don't have a lock_guard in
> scope when trying to call the non-atomic form, it won't compile.
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://gcc.gnu.org/pipermail/libstdc++/attachments/20260625/32773d63/attachment.htm>