On Thu, 29 Jun 2023 at 10:39, Jonathan Wakely wrote:
>
> On Thu, 29 Jun 2023 at 10:27, Alexey Lapshin
> <alexey.lapshin@xxxxxxxxxxxxx> wrote:
> >
> > On Thu, 2023-06-29 at 10:23 +0100, Jonathan Wakely wrote:
> > > That macro isn't even defined until later in the file, so would never
> > > be defined at this point.
> >
> > Was a simplified example.
> >
> >
> > Why do you still need to have some mutexes or whatever in case TZDB_DISABLED?
>
> Because there is still a global object that needs to be lazily
> constructed, and multiple threads can try to access it concurrently.
The attached patch should work for your target.
commit 881299967d8778e29f803d8801af9a0b1a5bd1bd
Author: Jonathan Wakely <jwakely@xxxxxxxxxx>
Date: Thu Jun 29 11:40:32 2023
libstdc++: Fix src/c++20/tzdb.cc for non-constexpr std::mutex
Building libstdc++ reportedly fails for targets without lock-free
std::atomic<T*> which don't define __GTHREAD_MUTEX_INIT:
src/c++20/tzdb.cc:110:21: error: 'constinit' variable 'std::chrono::{anonymous}::list_mutex' does not have a constant initializer
src/c++20/tzdb.cc:110:21: error: call to non-'constexpr' function 'std::mutex::mutex()'
The solution implemented by this commit is to use a local static mutex
when it can't be constinit, so that it's constructed on first use.
With this change, we can also simplify the preprocessor logic for
defining USE_ATOMIC_SHARED_PTR. It now depends on the same conditions as
USE_ATOMIC_LIST_HEAD, so in theory we could have a single macro. Keeping
them separate would allow us to replace the use of atomic<shared_ptr<T>>
with a mutex if that performs better, without having to give up on the
lock-free cache for fast access to the list head.
libstdc++-v3/ChangeLog:
* src/c++20/tzdb.cc (USE_ATOMIC_SHARED_PTR): Define consistently
with USE_ATOMIC_LIST_HEAD.
(list_mutex): Replace global object with function. Use local
static object when std::mutex constructor isn't constexpr.
diff --git a/libstdc++-v3/src/c++20/tzdb.cc b/libstdc++-v3/src/c++20/tzdb.cc
index a43b4f33eba..8d27726016e 100644
--- a/libstdc++-v3/src/c++20/tzdb.cc
+++ b/libstdc++-v3/src/c++20/tzdb.cc
@@ -41,22 +41,17 @@
# include <cstdlib> // getenv
#endif
-#ifndef __GTHREADS
-# define USE_ATOMIC_SHARED_PTR 0
-#elif _WIN32
-// std::mutex cannot be constinit, so Windows must use atomic<shared_ptr<>>.
-# define USE_ATOMIC_SHARED_PTR 1
-#elif ATOMIC_POINTER_LOCK_FREE < 2
-# define USE_ATOMIC_SHARED_PTR 0
-#else
-// TODO benchmark atomic<shared_ptr<>> vs mutex.
-# define USE_ATOMIC_SHARED_PTR 1
-#endif
-
#if defined __GTHREADS && ATOMIC_POINTER_LOCK_FREE == 2
# define USE_ATOMIC_LIST_HEAD 1
+// TODO benchmark atomic<shared_ptr<>> vs mutex.
+# define USE_ATOMIC_SHARED_PTR 1
#else
# define USE_ATOMIC_LIST_HEAD 0
+# define USE_ATOMIC_SHARED_PTR 0
+#endif
+
+#if USE_ATOMIC_SHARED_PTR && ! USE_ATOMIC_LIST_HEAD
+# error Unsupported combination
#endif
#if ! __cpp_constinit
@@ -106,9 +101,18 @@ namespace std::chrono
// Dummy no-op mutex type for single-threaded targets.
struct mutex { void lock() { } void unlock() { } };
#endif
- /// XXX std::mutex::mutex() not constexpr on Windows, so can't be constinit
- constinit mutex list_mutex;
+ inline mutex& list_mutex()
+ {
+#ifdef __GTHREAD_MUTEX_INIT
+ constinit static mutex m;
+#else
+ // Cannot use a constinit mutex, so use a local static.
+ alignas(mutex) constinit static char buf[sizeof(mutex)];
+ static mutex& m = *::new(buf) mutex();
#endif
+ return m;
+ }
+#endif // ! USE_ATOMIC_SHARED_PTR
struct Rule;
}
@@ -154,7 +158,7 @@ namespace std::chrono
static _Node*
_S_list_head(memory_order)
{
- lock_guard<mutex> l(list_mutex);
+ lock_guard<mutex> l(list_mutex());
return _S_head_owner.get();
}
@@ -1279,7 +1283,7 @@ namespace std::chrono
}
// XXX small window here where _S_head_cache still points to previous tzdb.
#else
- lock_guard<mutex> l(list_mutex);
+ lock_guard<mutex> l(list_mutex());
if (const _Node* h = _S_head_owner.get())
{
if (h->db.version == new_head_ptr->db.version)
@@ -1406,11 +1410,12 @@ namespace std::chrono
#else
if (Node::_S_list_head(memory_order::relaxed) != nullptr) [[likely]]
{
- lock_guard<mutex> l(list_mutex);
+ lock_guard<mutex> l(list_mutex());
const tzdb& current = Node::_S_head_owner->db;
if (current.version == version)
return current;
}
+ shared_ptr<Node> head; // Passed as unused arg to _S_replace_head.
#endif
auto [leaps, leaps_ok] = Node::_S_read_leap_seconds();
@@ -1499,9 +1504,6 @@ namespace std::chrono
ranges::sort(node->db.links, {}, &time_zone_link::name);
ranges::stable_sort(node->rules, {}, &Rule::name);
-#if ! USE_ATOMIC_SHARED_PTR
- shared_ptr<Node> head;
-#endif
return Node::_S_replace_head(std::move(head), std::move(node));
#else
__throw_disabled();
@@ -1526,7 +1528,7 @@ namespace std::chrono
#if USE_ATOMIC_SHARED_PTR
return const_iterator{_Node::_S_head_owner.load()};
#else
- lock_guard<mutex> l(list_mutex);
+ lock_guard<mutex> l(list_mutex());
return const_iterator{_Node::_S_head_owner};
#endif
}
@@ -1539,7 +1541,7 @@ namespace std::chrono
if (p._M_node) [[likely]]
{
#if ! USE_ATOMIC_SHARED_PTR
- lock_guard<mutex> l(list_mutex);
+ lock_guard<mutex> l(list_mutex());
#endif
if (auto next = p._M_node->next) [[likely]]
return const_iterator{p._M_node->next = std::move(next->next)};
