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: #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. 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. return new_head_ptr->db; > > libstdc++-v3/ChangeLog: > > * src/c++20/tzdb.cc (_Node::NumLeapSeconds): New class. > (_Node::num_leap_seconds): New static variable. > (num_leap_seconds): Remove. > (__detail::__recent_leap_second_info): Replace uses of > num_leap_seconds with _Node::num_leap_seconds. > (_Node::_S_replace_head): Likewise. > --- > > Tested x86_64-linux. > > libstdc++-v3/src/c++20/tzdb.cc | 106 ++++++++++++++++++++++++++------- > 1 file changed, 85 insertions(+), 21 deletions(-) > > diff --git a/libstdc++-v3/src/c++20/tzdb.cc > b/libstdc++-v3/src/c++20/tzdb.cc > index 5793155b6d89..17980b6e92ce 100644 > --- a/libstdc++-v3/src/c++20/tzdb.cc > +++ b/libstdc++-v3/src/c++20/tzdb.cc > @@ -67,6 +67,7 @@ > #endif > > #if USE_ATOMIC_SHARED_PTR && ! USE_ATOMIC_LIST_HEAD > +// Cannot use atomic<shared_ptr<T>> without lock-free atomic<T*>. > # error Unsupported combination > #endif > > @@ -200,6 +201,10 @@ namespace std::chrono > > // This is here because _Node is a friend so can call private > constructor. > static const leap_second fixed_leaps[]; > + > + // This is a member so that it can access fixed_leaps. > + struct NumLeapSeconds; > + static NumLeapSeconds num_leap_seconds; > }; > > // Implementation of the private constructor used for the singleton > object. > @@ -1301,12 +1306,83 @@ namespace > // The expiry date corresponding to the list above. > // tzdata 2026a leapseconds list expires at 2026-12-28 00:00:00 UTC > constexpr seconds fixed_expiry{1798416000u}; > - > - // This holds the most up-to-date number of leap seconds known at > runtime. > - // Initially zero, updated when _S_read_leap_seconds() is called. > - constinit atomic<unsigned> num_leap_seconds{0}; > } > > +// This holds the most up-to-date number of leap seconds known at runtime. > +// Initially zero, updated when _S_read_leap_seconds() is called. > +struct tzdb_list::_Node::NumLeapSeconds > +{ > + // Called by __recent_leap_second_info to read num_leap_seconds. > + unsigned > + get() > + { > +#if ATOMIC_INT_LOCK_FREE == 2 > + atomic_ref<unsigned> ref(count); > + auto num = ref.load(memory_order::relaxed); > + > + if (num == std::size(_Node::fixed_leaps)) > + // A leapseconds file has been read and has no new leap seconds. > + return num; > + > + if (num == 0) > + // No leapseconds file has been read yet. > + return 0; > + > + // The tzdb_list has been initialized and contains a tzdb object with > + // new leap seconds, which the caller is going to use. > + // The relaxed load above does not synchronize with anything, so to > + // ensure that the get_tzdb_list() in the caller will see a tzdb > object > + // set by _S_replace_head, we load num_leap_seconds again with acquire > + // ordering: > + return ref.load(memory_order::acquire); > This leads to improvement in readability, so I think we benefited from the change. > +#else > + lock_guard<mutex> l(list_mutex()); // This ensures acquire ordering. > + return count; > +#endif > + } > + > + // Called by __recent_leap_second_info to set num_leap_seconds when > + // we have determined there are no new leap seconds in a leapseconds > file. > + void > + set_to_fixed_size() > Would split this method instead, as decribed in above. > + { > +#if ATOMIC_INT_LOCK_FREE == 2 > + atomic_ref<unsigned> ref(count); > + unsigned expected = 0; > + ref.compare_exchange_strong(expected, std::size(_Node::fixed_leaps), > + memory_order::relaxed); > +#else + lock_guard<mutex> l(list_mutex()); > + if (count == 0) > + count = std::size(_Node::fixed_leaps); > +#endif > + } > + > + // Called by _Node::_S_replace_head > + void > + 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); > +#elif USE_ATOMIC_SHARED_PTR > + // The caller doesn't lock list_mutex() in this case, but it doesn't > seem > + // realistic to have lock-free atomic pointers and needs a lock for > int. > +# error Unsupported combination > +#else > + // XXX The only caller of this function locks list_mutex() so we would > + // deadlock if we locked it again here. > + count = val; > +#endif > + } > + > +private: > + unsigned count = 0; > +}; > + > +constinit tzdb_list::_Node::NumLeapSeconds > tzdb_list::_Node::num_leap_seconds; > + > namespace __detail > { > // Called by chrono::__detail::__get_leap_second_info in <chrono> > @@ -1368,19 +1444,11 @@ namespace > > constexpr auto num_fixed_leaps = std::size(_Node::fixed_leaps); > > - auto num_leaps = num_leap_seconds.load(memory_order::relaxed); > + auto num_leaps = _Node::num_leap_seconds.get(); > if (num_leaps == num_fixed_leaps) > // A leapseconds file has been read and has no new leap seconds: > return update_info(_Node::fixed_leaps); > - else if (num_leaps != 0) > - // The tzdb_list has been initialized and contains a tzdb object > - // with new leap seconds, which we want to use here. > - // The relaxed load above does not synchronize with anything, so to > - // ensure that the get_tzdb_list() below will see a tzdb object set > - // by _S_replace_head, we load num_leap_seconds again with acquire > - // ordering: > - (void) num_leap_seconds.load(memory_order::acquire); > - else > + else if (num_leaps == 0) > { > // The tzdb_list has not been initialized yet, so we don't know > // the correct number of leap seconds. > @@ -1389,11 +1457,9 @@ namespace > // to parse all of tzdata.zi and initialize a whole tzdb object. > if (_Node::_S_read_leap_seconds().first.size() == > num_fixed_leaps) > { > - // There are no new leap seconds. remember that so that the > next > + // There are no new leap seconds. Remember that so that the > next > // call to this function can just use fixed_leaps. > - num_leap_seconds.compare_exchange_strong(num_leaps, > - num_fixed_leaps, > - > memory_order::relaxed); > + _Node::num_leap_seconds.set_to_fixed_size(); > return update_info(_Node::fixed_leaps); > } > // else there are new leap seconds. We init tzdb_list so that the > @@ -1539,9 +1605,7 @@ namespace > > // This allows __recent_leap_second_info() to know that it can use > // get_tzdb_list()->begin()->leap_seconds to get new leap seconds. > - // The release op here synchronizes with the acquire op there. > - num_leap_seconds.store(new_head_ptr->db.leap_seconds.size(), > - memory_order::release); > + num_leap_seconds.set(new_head_ptr->db.leap_seconds.size()); > > return new_head_ptr->db; > } > -- > 2.54.0 > >
