On Thu, Jun 25, 2026 at 2:17 PM Jonathan Wakely <[email protected]> wrote:
> On Thu, 25 Jun 2026 at 13:08, Jonathan Wakely <[email protected]> wrote: > > > > On Thu, 25 Jun 2026 at 13:01, Tomasz Kaminski <[email protected]> > wrote: > > > > > > > > > > > > On Thu, Jun 25, 2026 at 1:42 PM Jonathan Wakely <[email protected]> > 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. > >
