[tzdb] Replace shared_mutex with mutex. (#87929)

The overhead of taking a std::mutex is much lower than taking a reader
lock on a shared mutex, even under heavy contention.

The benefit of shared_mutex only occurs as the amount of
time spent in the critical sections grows large enough.

In our case all we do is read a pointer and return the lock.
As a result, using a shared lock can be ~50%-100% slower

Here are the results for the provided benchmark on my machine:

```
2024-04-07T12:48:51-04:00
Running ./libcxx/benchmarks/shared_mutex_vs_mutex.libcxx.out
Run on (12 X 400 MHz CPU s)
CPU Caches:
  L1 Data 32 KiB (x6)
  L1 Instruction 32 KiB (x6)
  L2 Unified 1024 KiB (x6)
  L3 Unified 32768 KiB (x1)
Load Average: 2.70, 2.70, 1.63
---------------------------------------------------------------------
Benchmark                           Time             CPU   Iterations
---------------------------------------------------------------------
BM_shared_mutex/threads:1        13.9 ns         13.9 ns     50533700
BM_shared_mutex/threads:2        34.5 ns         68.9 ns      9957784
BM_shared_mutex/threads:4        38.4 ns          137 ns      4987772
BM_shared_mutex/threads:8        51.1 ns          358 ns      1974160
BM_shared_mutex/threads:32       57.1 ns          682 ns      1043648
BM_mutex/threads:1               5.54 ns         5.53 ns    125867422
BM_mutex/threads:2               15.5 ns         30.9 ns     21830116
BM_mutex/threads:4               15.4 ns         57.2 ns     12136920
BM_mutex/threads:8               19.3 ns          140 ns      4997080
BM_mutex/threads:32              20.8 ns          252 ns      2859808
```

libcxx/benchmarks/CMakeLists.txt

@@ -221,6 +221,7 @@ set(BENCHMARK_TESTS
     map.bench.cpp
     monotonic_buffer.bench.cpp
     ordered_set.bench.cpp
+    shared_mutex_vs_mutex.bench.cpp
     stop_token.bench.cpp
     std_format_spec_string_unicode.bench.cpp
     string.bench.cpp

libcxx/benchmarks/shared_mutex_vs_mutex.bench.cpp

@@ -0,0 +1,41 @@
+//===----------------------------------------------------------------------===//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// This benchmark compares the performance of std::mutex and std::shared_mutex in contended scenarios.
+// it's meant to establish a baseline overhead for std::shared_mutex and std::mutex, and to help inform decisions about
+// which mutex to use when selecting a mutex type for a given use case.
+
+#include <atomic>
+#include <mutex>
+#include <numeric>
+#include <shared_mutex>
+#include <thread>
+
+#include "benchmark/benchmark.h"
+
+int global_value = 42;
+std::mutex m;
+std::shared_mutex sm;
+
+static void BM_shared_mutex(benchmark::State& state) {
+  for (auto _ : state) {
+    std::shared_lock<std::shared_mutex> lock(sm);
+    benchmark::DoNotOptimize(global_value);
+  }
+}
+
+static void BM_mutex(benchmark::State& state) {
+  for (auto _ : state) {
+    std::lock_guard<std::mutex> lock(m);
+    benchmark::DoNotOptimize(global_value);
+  }
+}
+
+BENCHMARK(BM_shared_mutex)->Threads(1)->Threads(2)->Threads(4)->Threads(8)->Threads(32);
+BENCHMARK(BM_mutex)->Threads(1)->Threads(2)->Threads(4)->Threads(8)->Threads(32);
+
+BENCHMARK_MAIN();

libcxx/src/include/tzdb/tzdb_list_private.h

@@ -15,8 +15,11 @@
 #include <forward_list>
 
 // When threads are not available the locking is not required.
+// When threads are available, we use std::mutex over std::shared_mutex
+// due to the increased overhead of std::shared_mutex.
+// See shared_mutex_vs_mutex.bench.cpp
 #ifndef _LIBCPP_HAS_NO_THREADS
-#  include <shared_mutex>
+#  include <mutex>
 #endif
 
 #include "types_private.h"
@@ -56,7 +59,7 @@ class tzdb_list::__impl {
 
   const tzdb& __front() const noexcept {
 #ifndef _LIBCPP_HAS_NO_THREADS
-    shared_lock __lock{__mutex_};
+    unique_lock __lock{__mutex_};
 #endif
     return __tzdb_.front();
   }
@@ -72,7 +75,7 @@ class tzdb_list::__impl {
 
   const_iterator __begin() const noexcept {
 #ifndef _LIBCPP_HAS_NO_THREADS
-    shared_lock __lock{__mutex_};
+    unique_lock __lock{__mutex_};
 #endif
     return __tzdb_.begin();
   }
@@ -87,7 +90,7 @@ class tzdb_list::__impl {
   void __load_no_lock() { chrono::__init_tzdb(__tzdb_.emplace_front(), __rules_.emplace_front()); }
 
 #ifndef _LIBCPP_HAS_NO_THREADS
-  mutable shared_mutex __mutex_;
+  mutable mutex __mutex_;
 #endif
   forward_list<tzdb> __tzdb_;