libcxx/test/std/thread/thread.mutex/thread.mutex.requirements/thread.sharedtimedmutex.requirements/thread.sharedtimedmutex.class/lock_shared.pass.cpp

   1 //===----------------------------------------------------------------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8
   9 // UNSUPPORTED: no-threads
  10 // UNSUPPORTED: c++03, c++11
  11
  12 // <shared_mutex>
  13
  14 // class shared_timed_mutex;
  15
  16 // void lock_shared();
  17
  18 #include <shared_mutex>
  19 #include <algorithm>
  20 #include <atomic>
  21 #include <cassert>
  22 #include <thread>
  23 #include <vector>
  24
  25 #include "make_test_thread.h"
  26
  27 int main(int, char**) {
  28   // Lock-shared a mutex that is not locked yet. This should succeed.
  29   {
  30     std::shared_timed_mutex m;
  31     std::vector<std::thread> threads;
  32     for (int i = 0; i != 5; ++i) {
  33       threads.push_back(support::make_test_thread([&] {
  34         m.lock_shared();
  35         m.unlock_shared();
  36       }));
  37     }
  38
  39     for (auto& t : threads)
  40       t.join();
  41   }
  42
  43   // Lock-shared a mutex that is already exclusively locked. This should block until it is unlocked.
  44   {
  45     std::atomic<int> ready(0);
  46     std::shared_timed_mutex m;
  47     m.lock();
  48     std::atomic<bool> is_locked_from_main(true);
  49
  50     std::vector<std::thread> threads;
  51     for (int i = 0; i != 5; ++i) {
  52       threads.push_back(support::make_test_thread([&] {
  53         ++ready;
  54         while (ready < 5)
  55           /* wait until all threads have been created */;
  56
  57         m.lock_shared();
  58         assert(!is_locked_from_main);
  59         m.unlock_shared();
  60       }));
  61     }
  62
  63     while (ready < 5)
  64       /* wait until all threads have been created */;
  65
  66     // We would rather signal this after we unlock, but that would create a race condition.
  67     // We instead signal it before we unlock, which means that it's technically possible for
  68     // the thread to take the lock while we're still holding it and for the test to still pass.
  69     is_locked_from_main = false;
  70     m.unlock();
  71
  72     for (auto& t : threads)
  73       t.join();
  74   }
  75
  76   // Lock-shared a mutex that is already lock-shared. This should succeed.
  77   {
  78     std::atomic<int> ready(0);
  79     std::shared_timed_mutex m;
  80     m.lock_shared();
  81
  82     std::vector<std::thread> threads;
  83     for (int i = 0; i != 5; ++i) {
  84       threads.push_back(support::make_test_thread([&] {
  85         ++ready;
  86         while (ready < 5)
  87           /* wait until all threads have been created */;
  88
  89         m.lock_shared();
  90         m.unlock_shared();
  91       }));
  92     }
  93
  94     while (ready < 5)
  95       /* wait until all threads have been created */;
  96
  97     m.unlock_shared();
  98
  99     for (auto& t : threads)
 100       t.join();
 101   }
 102
 103   // Create several threads that all acquire-shared the same mutex and make sure that each
 104   // thread successfully acquires-shared the mutex.
 105   //
 106   // We record how many other threads were holding the mutex when it was acquired, which allows
 107   // us to know whether the test was somewhat effective at causing multiple threads to lock at
 108   // the same time.
 109   {
 110     std::shared_timed_mutex mutex;
 111     std::vector<std::thread> threads;
 112     constexpr int n_threads = 5;
 113     std::atomic<int> holders(0);
 114     int concurrent_holders[n_threads] = {};
 115     std::atomic<bool> ready(false);
 116
 117     for (int i = 0; i != n_threads; ++i) {
 118       threads.push_back(support::make_test_thread([&, i] {
 119         while (!ready)
 120           /* spin */;
 121
 122         mutex.lock_shared();
 123         ++holders;
 124         concurrent_holders[i] = holders;
 125
 126         mutex.unlock_shared();
 127         --holders;
 128       }));
 129     }
 130
 131     ready = true; // let the threads actually start shared-acquiring the mutex
 132     for (auto& t : threads)
 133       t.join();
 134
 135     // We can't guarantee that we'll ever have more than 1 concurrent holder so that's what
 136     // we assert, however in principle we should often trigger more than 1 concurrent holder.
 137     int max_concurrent_holders = *std::max_element(std::begin(concurrent_holders), std::end(concurrent_holders));
 138     assert(max_concurrent_holders >= 1);
 139   }
 140
 141   return 0;
 142 }