Threads in thread pool for shared_work scheduler got stuck in suspend_until wait

I am writing a test to test shared_work scheduler with buffered_channel based on work_sharing.cpp example.

In the test, I have a thread pool with 8 threads, all of them are running shared_worker scheduler with suspend = True. There are two buffered channels to do synchronization between fibers

Two fibers are pushing input into channel 1 like this.
```C++
boost::fibers::fiber{[&chan1] {

int i = 0;
int counter{0};
while (true) {
char item = (i++%26)+'a';
chan1.push(item);
std::cout << "pushed " << item << " into pipeline" << std::endl;
auto prev_thread = std::this_thread::get_id();
boost::this_fiber::yield();
if (++counter == 1024) {
counter = 0;
boost::this_fiber::sleep_for(std::chrono::milliseconds(10+rand()%10));
}
auto after_thread = std::this_thread::get_id();
if (prev_thread != after_thread) {
std::cout << "feeder 1 migrated from " << prev_thread << " to "
<< after_thread << std::endl;
}
}
lock_type lk( mtx_count);
if ( 0 == --fiber_count) { /*< Decrement fiber counter for each completed fiber. >*/
lk.unlock();
cnd_count.notify_all(); /*< Notify all fibers waiting on `cnd_count`. >*/
}
}}.detach();
```
Four Fibers to pop from buffer channel 1 , do some CPU intensive task and push item into buffer channel 2 like this
``` C++
boost::fibers::fiber([&chan1, &chan2]() {

while (true) {
char item;
chan1.pop(item);
cout << "stage 1:2 consumed " << item << endl;
do_primes();
chan2.push(item);
auto prev_thread = std::this_thread::get_id();
boost::this_fiber::yield();
auto after_thread = std::this_thread::get_id();
if (prev_thread != after_thread) {
std::cout << "stage 1:2 migrated from " << prev_thread << " to "
<< after_thread << std::endl;
}

}
lock_type lk( mtx_count);
if ( 0 == --fiber_count) { /*< Decrement fiber counter for each completed fiber. >*/
lk.unlock();
cnd_count.notify_all(); /*< Notify all fibers waiting on `cnd_count`. >*/
}
}).detach();
```
Another four fibers to pop from buffer channel 2 and do some CPU intensive task like this
``` C++
boost::fibers::fiber([&chan2]() {

while (true) {
char item;
chan2.pop(item);
do_primes1();
cout << "stage 2:1 consumed " << item << endl;
auto prev_thread = std::this_thread::get_id();
boost::this_fiber::yield();
auto after_thread = std::this_thread::get_id();
if (prev_thread != after_thread) {
std::cout << "stage 2:1 migrated from " << prev_thread << " to "
<< after_thread << std::endl;
}

}
lock_type lk( mtx_count);
if ( 0 == --fiber_count) { /*< Decrement fiber counter for each completed fiber. >*/
lk.unlock();
cnd_count.notify_all(); /*< Notify all fibers waiting on `cnd_count`. >*/
}
}).detach();
```

When running the test on 8 core machine, I observed that some worker threads in thread pool are never running.

And when I gdb to look into stuck thread, it looks like they are blocked by wait in suspend_until

Each time I run the test, the number of threads that are stuck is random. I also try the test without buffered channel, the random disappears and everything seems perfect.
I have run the test on both libboost-fiber-1.71 and newest relase 1.75. The issue is still there.

Is there a race condition for conditional_variable in scheduler and buffered channel that may cause this issue? Is there any workaround to avoid it?
Thank you!

Here is the complete test code to reproduce the issue
```
// Copyright Nat Goodspeed + Oliver Kowalke 2015.
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE_1_0.txt or copy at
// http://www.boost.org/LICENSE_1_0.txt)

#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include

static std::size_t fiber_count{ 0 };
static std::mutex mtx_count{};
static boost::fibers::condition_variable_any cnd_count{};
typedef std::unique_lock lock_type;
using boost::fibers::detail::thread_barrier;

const int MAX_PRIME = 10000;

void do_primes()
{
unsigned long i, num, primes = 0;
for (num = 1; num <= MAX_PRIME; ++num) {
for (i = 2; (i <= num) && (num % i != 0); ++i);
if (i == num)
++primes;
}
printf("Calculated %d primes 0.\n", primes);
}

void do_primes1()
{
unsigned long i, num, primes = 0;
for (num = 1; num <= MAX_PRIME; ++num) {
for (i = 2; (i <= num) && (num % i != 0); ++i);
if (i == num)
++primes;
}
printf("Calculated %d primes 1.\n", primes);
}

/*****************************************************************************
* example fiber function
*****************************************************************************/

/*****************************************************************************
* example thread function
*****************************************************************************/

void start_thread( thread_barrier * b, int i) {
std::ostringstream buffer;
std::string thread_name = std::string{"worker_thread_"} + std::to_string(i);
pthread_setname_np(pthread_self(), thread_name.c_str()); // set the name (pthread_self() returns the pthread_t of the current thread)
buffer << "thread started " << std::this_thread::get_id() << std::endl;
boost::fibers::use_scheduling_algorithm< boost::fibers::algo::shared_work>(true); /*<
Install the scheduling algorithm `boost::fibers::algo::shared_work` in order to
join the work sharing.
>*/
b->wait(); /*< sync with other threads: allow them to start processing >*/
boost::fibers::fiber test_fiber([] {
while(true) {
boost::this_fiber::sleep_for(std::chrono::seconds(1));
}
});
lock_type lk( mtx_count);
std::cout << buffer.str() << std::flush;
cnd_count.wait( lk, [](){return 0 == fiber_count;} ); /*<
Suspend main fiber and resume worker fibers in the meanwhile.
Main fiber gets resumed (e.g returns from `condition_variable_any::wait()`)
if all worker fibers are complete.
>*/
test_fiber.join();
BOOST_ASSERT( 0 == fiber_count);
}
//]

/*****************************************************************************
* main()
*****************************************************************************/
using std::endl;
using std::cout;
int main( int argc, char *argv[]) {
//[main_ws
/*<
Install the scheduling algorithm `boost::fibers::algo::shared_work` in the main
thread too, so each new fiber gets launched into the shared pool.
>*/
boost::fibers::use_scheduling_algorithm(true);
boost::fibers::buffered_channel chan1(64);
boost::fibers::buffered_channel chan2(64);
boost::fibers::fiber([&chan1, &chan2]() {

while (true) {
char item;
chan1.pop(item);
cout << "stage 1:1 consumed " << item << endl;
do_primes();
chan2.push(item);
auto prev_thread = std::this_thread::get_id();
boost::this_fiber::yield();
auto after_thread = std::this_thread::get_id();
if (prev_thread != after_thread) {
std::cout << "stage 1:1 migrated from " << prev_thread << " to "
<< after_thread << std::endl;
}

}
lock_type lk( mtx_count);
if ( 0 == --fiber_count) { /*< Decrement fiber counter for each completed fiber. >*/
lk.unlock();
cnd_count.notify_all(); /*< Notify all fibers waiting on `cnd_count`. >*/
}
}).detach();
fiber_count ++;
boost::fibers::fiber([&chan1, &chan2]() {

while (true) {
char item;
chan1.pop(item);
cout << "stage 1:2 consumed " << item << endl;
do_primes();
chan2.push(item);
auto prev_thread = std::this_thread::get_id();
boost::this_fiber::yield();
auto after_thread = std::this_thread::get_id();
if (prev_thread != after_thread) {
std::cout << "stage 1:2 migrated from " << prev_thread << " to "
<< after_thread << std::endl;
}

}
lock_type lk( mtx_count);
if ( 0 == --fiber_count) { /*< Decrement fiber counter for each completed fiber. >*/
lk.unlock();
cnd_count.notify_all(); /*< Notify all fibers waiting on `cnd_count`. >*/
}
}).detach();
fiber_count ++;
boost::fibers::fiber([&chan1, &chan2]() {

while (true) {
char item;
chan1.pop(item);
cout << "stage 1:3 consumed " << item << endl;
do_primes();
chan2.push(item);
auto prev_thread = std::this_thread::get_id();
boost::this_fiber::yield();
auto after_thread = std::this_thread::get_id();
if (prev_thread != after_thread) {
std::cout << "stage 1:2 migrated from " << prev_thread << " to "
<< after_thread << std::endl;
}

}
lock_type lk( mtx_count);
if ( 0 == --fiber_count) { /*< Decrement fiber counter for each completed fiber. >*/
lk.unlock();
cnd_count.notify_all(); /*< Notify all fibers waiting on `cnd_count`. >*/
}
}).detach();
fiber_count ++;
boost::fibers::fiber([&chan1, &chan2]() {

while (true) {
char item;
chan1.pop(item);
cout << "stage 1:4 consumed " << item << endl;
do_primes();
chan2.push(item);
auto prev_thread = std::this_thread::get_id();
boost::this_fiber::yield();
auto after_thread = std::this_thread::get_id();
if (prev_thread != after_thread) {
std::cout << "stage 1:2 migrated from " << prev_thread << " to "
<< after_thread << std::endl;
}

}
lock_type lk( mtx_count);
if ( 0 == --fiber_count) { /*< Decrement fiber counter for each completed fiber. >*/
lk.unlock();
cnd_count.notify_all(); /*< Notify all fibers waiting on `cnd_count`. >*/
}
}).detach();
fiber_count ++;
boost::fibers::fiber([&chan2]() {

while (true) {
char item;
chan2.pop(item);
do_primes1();
cout << "stage 2:1 consumed " << item << endl;
auto prev_thread = std::this_thread::get_id();
boost::this_fiber::yield();
auto after_thread = std::this_thread::get_id();
if (prev_thread != after_thread) {
std::cout << "stage 2:1 migrated from " << prev_thread << " to "
<< after_thread << std::endl;
}

}
lock_type lk( mtx_count);
if ( 0 == --fiber_count) { /*< Decrement fiber counter for each completed fiber. >*/
lk.unlock();
cnd_count.notify_all(); /*< Notify all fibers waiting on `cnd_count`. >*/
}
}).detach();
fiber_count ++;
boost::fibers::fiber([&chan2]() {

while (true) {
char item;
chan2.pop(item);
do_primes1();
cout << "stage 2:2 consumed " << item << endl;
auto prev_thread = std::this_thread::get_id();
boost::this_fiber::yield();
auto after_thread = std::this_thread::get_id();
if (prev_thread != after_thread) {
std::cout << "stage 2:2 migrated from " << prev_thread << " to "
<< after_thread << std::endl;
}

}
lock_type lk( mtx_count);
if ( 0 == --fiber_count) { /*< Decrement fiber counter for each completed fiber. >*/
lk.unlock();
cnd_count.notify_all(); /*< Notify all fibers waiting on `cnd_count`. >*/
}
}).detach();
fiber_count ++;
boost::fibers::fiber([&chan2]() {

while (true) {
char item;
chan2.pop(item);
do_primes1();
cout << "stage 2:3 consumed " << item << endl;
auto prev_thread = std::this_thread::get_id();
boost::this_fiber::yield();
auto after_thread = std::this_thread::get_id();
if (prev_thread != after_thread) {
std::cout << "stage 2:2 migrated from " << prev_thread << " to "
<< after_thread << std::endl;
}

}
lock_type lk( mtx_count);
if ( 0 == --fiber_count) { /*< Decrement fiber counter for each completed fiber. >*/
lk.unlock();
cnd_count.notify_all(); /*< Notify all fibers waiting on `cnd_count`. >*/
}
}).detach();
fiber_count ++;
boost::fibers::fiber([&chan2]() {

while (true) {
char item;
chan2.pop(item);
do_primes1();
cout << "stage 2:4 consumed " << item << endl;
auto prev_thread = std::this_thread::get_id();
boost::this_fiber::yield();
auto after_thread = std::this_thread::get_id();
if (prev_thread != after_thread) {
std::cout << "stage 2:2 migrated from " << prev_thread << " to "
<< after_thread << std::endl;
}

}
lock_type lk( mtx_count);
if ( 0 == --fiber_count) { /*< Decrement fiber counter for each completed fiber. >*/
lk.unlock();
cnd_count.notify_all(); /*< Notify all fibers waiting on `cnd_count`. >*/
}
}).detach();
fiber_count ++;

boost::fibers::fiber{[&chan1] {

int i = 0;
int counter{0};
while (true) {
char item = (i++%26)+'a';
chan1.push(item);
std::cout << "pushed " << item << " into pipeline" << std::endl;
auto prev_thread = std::this_thread::get_id();
boost::this_fiber::yield();
if (++counter == 1024) {
counter = 0;
boost::this_fiber::sleep_for(std::chrono::milliseconds(10+rand()%10));
}
auto after_thread = std::this_thread::get_id();
if (prev_thread != after_thread) {
std::cout << "feeder 1 migrated from " << prev_thread << " to "
<< after_thread << std::endl;
}
}
lock_type lk( mtx_count);
if ( 0 == --fiber_count) { /*< Decrement fiber counter for each completed fiber. >*/
lk.unlock();
cnd_count.notify_all(); /*< Notify all fibers waiting on `cnd_count`. >*/
}
}}.detach();
fiber_count ++;
boost::fibers::fiber{[&chan1] {

int i = 0;
int counter{0};
while (true) {
char item = (i++%26)+'a';
chan1.push(item);
std::cout << "pushed " << item << " into pipeline" << std::endl;
auto prev_thread = std::this_thread::get_id();
boost::this_fiber::yield();
if (++counter == 1024) {
counter = 0;
boost::this_fiber::sleep_for(std::chrono::milliseconds(10+rand()%10));
}
auto after_thread = std::this_thread::get_id();
if (prev_thread != after_thread) {
std::cout << "feeder 2 migrated from " << prev_thread << " to "
<< after_thread << std::endl;
}
}
lock_type lk( mtx_count);
if ( 0 == --fiber_count) { /*< Decrement fiber counter for each completed fiber. >*/
lk.unlock();
cnd_count.notify_all(); /*< Notify all fibers waiting on `cnd_count`. >*/
}
}}.detach();
fiber_count ++;
thread_barrier b(8);
std::thread threads[] = {
std::thread(start_thread, &b, 1), std::thread(start_thread, &b, 5),
std::thread(start_thread, &b, 2), std::thread(start_thread, &b, 6),
std::thread(start_thread, &b, 3), std::thread(start_thread, &b, 7),
std::thread(start_thread, &b, 4)};
std::cout << "main thread started " << std::this_thread::get_id()
<< std::endl;
b.wait(); /*< sync with other threads: allow them to start processing >*/
{
lock_type /*< `lock_type` is typedef'ed as __unique_lock__<
[@http://en.cppreference.com/w/cpp/thread/mutex `std::mutex`] >
>*/
lk(mtx_count);
cnd_count.wait(lk, []() {return 0 == fiber_count;}); /*<
Suspend main fiber and resume worker fibers in the meanwhile.
Main fiber gets resumed (e.g returns from
`condition_variable_any::wait()`) if all worker fibers are complete.
>*/
}
BOOST_ASSERT(0 == fiber_count);
for (std::thread &t : threads) { /*< wait for threads to terminate >*/
t.join();
}
//]
std::cout << "done." << std::endl;
return EXIT_SUCCESS;
}
```