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 
11 // <condition_variable>
12 
13 // class condition_variable;
14 
15 // void notify_one();
16 
17 
18 // NOTE: `notify_one` is just a wrapper around pthread_cond_signal, but
19 // POSIX does not guarantee that one and only one thread will be woken:
20 //
21 // https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_cond_signal.html
22 //
23 // Quote:
24 //     Multiple Awakenings by Condition Signal
25 //     On a multi-processor, it may be impossible for an implementation of
26 //     pthread_cond_signal() to avoid the unblocking of more than one thread
27 //     blocked on a condition variable. For example...
28 
29 
30 
31 // NOTE: In previous versions of this test, `notify_one` was called WITHOUT
32 // holding the lock but POSIX says (in the aforementioned URL) that:
33 //     ...if predictable scheduling behavior is required, then that mutex shall
34 //     be locked by the thread calling pthread_cond_broadcast() or
35 //     pthread_cond_signal().
36 
37 
38 #include <condition_variable>
39 #include <atomic>
40 #include <mutex>
41 #include <thread>
42 #include <cassert>
43 
44 #include "make_test_thread.h"
45 #include "test_macros.h"
46 
47 
48 std::condition_variable cv;
49 std::mutex mut;
50 
51 std::atomic_int test1(0);
52 std::atomic_int test2(0);
53 std::atomic_int ready(2);
54 std::atomic_int which(0);
55 
f1()56 void f1()
57 {
58   std::unique_lock<std::mutex> lk(mut);
59   assert(test1 == 0);
60   --ready;
61   while (test1 == 0)
62     cv.wait(lk);
63   which = 1;
64   assert(test1 == 1);
65   test1 = 2;
66 }
67 
f2()68 void f2()
69 {
70   std::unique_lock<std::mutex> lk(mut);
71   assert(test2 == 0);
72   --ready;
73   while (test2 == 0)
74     cv.wait(lk);
75   which = 2;
76   assert(test2 == 1);
77   test2 = 2;
78 }
79 
main(int,char **)80 int main(int, char**)
81 {
82   std::thread t1 = support::make_test_thread(f1);
83   std::thread t2 = support::make_test_thread(f2);
84   {
85     while (ready > 0)
86       std::this_thread::yield();
87     // At this point:
88     // 1) Both f1 and f2 have entered their condition variable wait.
89     // 2) Either f1 or f2 has the mutex locked and is about to wait.
90     std::unique_lock<std::mutex> lk(mut);
91     test1 = 1;
92     test2 = 1;
93     ready = 1;
94     cv.notify_one();
95   }
96   {
97     while (which == 0)
98       std::this_thread::yield();
99     std::unique_lock<std::mutex> lk(mut);
100     if (test1 == 2) {
101       assert(test2 == 1);
102       t1.join();
103       test1 = 0;
104     } else {
105       assert(test1 == 1);
106       assert(test2 == 2);
107       t2.join();
108       test2 = 0;
109     }
110     which = 0;
111     cv.notify_one();
112   }
113   {
114     while (which == 0)
115       std::this_thread::yield();
116     std::unique_lock<std::mutex> lk(mut);
117     if (test1 == 2) {
118       assert(test2 == 0);
119       t1.join();
120       test1 = 0;
121     } else {
122       assert(test1 == 0);
123       assert(test2 == 2);
124       t2.join();
125       test2 = 0;
126     }
127   }
128 
129   return 0;
130 }
131