131 lines
3.0 KiB
C++
131 lines
3.0 KiB
C++
//===----------------------------------------------------------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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//
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// UNSUPPORTED: no-threads
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// <condition_variable>
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// class condition_variable;
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// void notify_one();
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// NOTE: `notify_one` is just a wrapper around pthread_cond_signal, but
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// POSIX does not guarantee that one and only one thread will be woken:
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//
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// https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_cond_signal.html
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//
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// Quote:
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// Multiple Awakenings by Condition Signal
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// On a multi-processor, it may be impossible for an implementation of
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// pthread_cond_signal() to avoid the unblocking of more than one thread
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// blocked on a condition variable. For example...
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// NOTE: In previous versions of this test, `notify_one` was called WITHOUT
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// holding the lock but POSIX says (in the aforementioned URL) that:
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// ...if predictable scheduling behavior is required, then that mutex shall
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// be locked by the thread calling pthread_cond_broadcast() or
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// pthread_cond_signal().
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#include <condition_variable>
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#include <atomic>
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#include <mutex>
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#include <thread>
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#include <cassert>
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#include "make_test_thread.h"
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#include "test_macros.h"
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std::condition_variable cv;
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std::mutex mut;
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std::atomic_int test1(0);
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std::atomic_int test2(0);
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std::atomic_int ready(2);
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std::atomic_int which(0);
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void f1()
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{
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std::unique_lock<std::mutex> lk(mut);
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assert(test1 == 0);
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--ready;
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while (test1 == 0)
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cv.wait(lk);
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which = 1;
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assert(test1 == 1);
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test1 = 2;
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}
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void f2()
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{
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std::unique_lock<std::mutex> lk(mut);
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assert(test2 == 0);
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--ready;
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while (test2 == 0)
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cv.wait(lk);
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which = 2;
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assert(test2 == 1);
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test2 = 2;
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}
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int main(int, char**)
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{
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std::thread t1 = support::make_test_thread(f1);
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std::thread t2 = support::make_test_thread(f2);
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{
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while (ready > 0)
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std::this_thread::yield();
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// At this point:
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// 1) Both f1 and f2 have entered their condition variable wait.
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// 2) Either f1 or f2 has the mutex locked and is about to wait.
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std::unique_lock<std::mutex> lk(mut);
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test1 = 1;
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test2 = 1;
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ready = 1;
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cv.notify_one();
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}
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{
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while (which == 0)
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std::this_thread::yield();
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std::unique_lock<std::mutex> lk(mut);
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if (test1 == 2) {
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assert(test2 == 1);
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t1.join();
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test1 = 0;
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} else {
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assert(test1 == 1);
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assert(test2 == 2);
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t2.join();
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test2 = 0;
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}
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which = 0;
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cv.notify_one();
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}
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{
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while (which == 0)
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std::this_thread::yield();
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std::unique_lock<std::mutex> lk(mut);
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if (test1 == 2) {
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assert(test2 == 0);
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t1.join();
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test1 = 0;
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} else {
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assert(test1 == 0);
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assert(test2 == 2);
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t2.join();
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test2 = 0;
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}
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}
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return 0;
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}
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