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, c++14
11 
12 // <shared_mutex>
13 
14 // class shared_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_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_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_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_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 
123         mutex.lock_shared();
124         ++holders;
125         concurrent_holders[i] = holders;
126 
127         mutex.unlock_shared();
128         --holders;
129       }));
130     }
131 
132     ready = true; // let the threads actually start shared-acquiring the mutex
133     for (auto& t : threads)
134       t.join();
135 
136     // We can't guarantee that we'll ever have more than 1 concurrent holder so that's what
137     // we assert, however in principle we should often trigger more than 1 concurrent holder.
138     int max_concurrent_holders = *std::max_element(std::begin(concurrent_holders), std::end(concurrent_holders));
139     assert(max_concurrent_holders >= 1);
140   }
141 
142   return 0;
143 }
144