// void notify_one();
+
+// NOTE: `notify_one` is just a wrapper around pthread_cond_signal, but
+// POSIX does not guarantee that one and only one thread will be woken:
+//
+// https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_cond_signal.html
+//
+// Quote:
+// Multiple Awakenings by Condition Signal
+// On a multi-processor, it may be impossible for an implementation of
+// pthread_cond_signal() to avoid the unblocking of more than one thread
+// blocked on a condition variable. For example...
+
+
+
+// NOTE: In previous versions of this test, `notify_one` was called WITHOUT
+// holding the lock but POSIX says (in the aforementioned URL) that:
+// ...if predictable scheduling behavior is required, then that mutex shall
+// be locked by the thread calling pthread_cond_broadcast() or
+// pthread_cond_signal().
+
+
#include <condition_variable>
#include <atomic>
#include <mutex>
void f1()
{
- --ready;
std::unique_lock<std::mutex> lk(mut);
assert(test1 == 0);
+ --ready;
while (test1 == 0)
cv.wait(lk);
which = 1;
void f2()
{
- --ready;
std::unique_lock<std::mutex> lk(mut);
assert(test2 == 0);
+ --ready;
while (test2 == 0)
cv.wait(lk);
which = 2;
{
std::thread t1(f1);
std::thread t2(f2);
- while (ready > 0)
- std::this_thread::yield();
- // In case the threads were preempted right after the atomic decrement but
- // before cv.wait(), we yield one more time.
- std::this_thread::yield();
{
- std::unique_lock<std::mutex>lk(mut);
+ while (ready > 0)
+ std::this_thread::yield();
+ // At this point:
+ // 1) Both f1 and f2 have entered their condition variable wait.
+ // 2) Either f1 or f2 has the mutex locked and is about to wait.
+ std::unique_lock<std::mutex> lk(mut);
test1 = 1;
test2 = 1;
ready = 1;
+ cv.notify_one();
}
- cv.notify_one();
{
while (which == 0)
std::this_thread::yield();
- std::unique_lock<std::mutex>lk(mut);
- }
- if (test1 == 2) {
- assert(test2 == 1);
- t1.join();
- test1 = 0;
- } else {
- assert(test1 == 1);
- assert(test2 == 2);
- t2.join();
- test2 = 0;
+ std::unique_lock<std::mutex> lk(mut);
+ if (test1 == 2) {
+ assert(test2 == 1);
+ t1.join();
+ test1 = 0;
+ } else {
+ assert(test1 == 1);
+ assert(test2 == 2);
+ t2.join();
+ test2 = 0;
+ }
+ which = 0;
+ cv.notify_one();
}
- which = 0;
- cv.notify_one();
{
while (which == 0)
std::this_thread::yield();
- std::unique_lock<std::mutex>lk(mut);
- }
- if (test1 == 2) {
- assert(test2 == 0);
- t1.join();
- test1 = 0;
- } else {
- assert(test1 == 0);
- assert(test2 == 2);
- t2.join();
- test2 = 0;
+ std::unique_lock<std::mutex> lk(mut);
+ if (test1 == 2) {
+ assert(test2 == 0);
+ t1.join();
+ test1 = 0;
+ } else {
+ assert(test1 == 0);
+ assert(test2 == 2);
+ t2.join();
+ test2 = 0;
+ }
}
return 0;