--- Begin Message ---
- To: Debian Bug Tracking System <submit@bugs.debian.org>
- Subject: threads: condition_variable or atomic causes _sometimes_ a deadlock
- From: "U.Mutlu" <um4711@mutluit.com>
- Date: Wed, 13 Jan 2016 02:44:57 +0100
- Message-id: <20160113014457.10540.86849.reportbug@c22-local>
Package: g++-4.9
Version: 4.9.2-10
Severity: normal
Dear Maintainer,
there is a mysterious bug in g++ or its libraries.
The bug happens when one compiles and executes the following test code multiple
times:
sometimes the executable just hangs, ie. a deadlock happens.
I think the error lies not in the test code, but in the g++ libraries or even
in g++ itself.
/*
test.cpp
2016-01-13-We
Author: U.Mutlu
condition variable example (but is [sometimes] buggy)
Code adapted from the example at [1]
Question:
How to fix this code?
--> it could be a low-level issue, ie. compiler-, stdlibrary-, or pthread-
issue
What-it-does:
10 threads race for a job. The main thread supplies the job.
One of the threads takes the job and processes it.
This is repeated 10000 times.
BUT: sometimes the pgm freezes (ie. a deadlock bug happens) --> see below
Compile:
g++ -Wall -O2 -std=gnu++11 test.cpp -lpthread
Run:
./a.out
repeat multiple times (up to 10 times should be ok for the runtime bug to
show up)
Symptoms of the bug:
sometimes pgm lands in an endless-loop! ie. deadlock happens --> pgm not
finishing
Test environment:
- g++ --version
g++ (Debian 4.9.2-10) 4.9.2
- uname -a
Linux my 4.2.0-0.bpo.1-amd64 #1 SMP Debian 4.2.6-3~bpo8+2 (2015-12-14)
x86_64 GNU/Linux
- dpkg -l | grep -i pthread
ii libpthread-stubs0-dev:amd64 0.3-4 amd64 pthread stubs not provided by
native libc, development files
See also:
[1] http://www.cplusplus.com/reference/condition_variable/condition_variable/
[2] http://en.cppreference.com/w/cpp/thread/condition_variable
*/
#include <iostream>
#include <atomic>
#include <thread>
#include <mutex>
#include <condition_variable>
#include <unistd.h>
#include <vector>
using namespace std;
mutex mtx;
condition_variable cv;
atomic<bool> fReady; // init done in main
atomic<bool> fQuit; // init done in main
atomic<bool> fThreadsOk; // init done in main
void threadfunc(int id)
{
/*
Any thread that intends to wait on condition_variable has to acquire a
unique_lock<mutex> first.
The wait operations atomically release the mutex and suspend the execution of
the thread.
When the condition variable is notified, the thread is awakened, and the mutex
is reacquired [2].
*/
while (!fQuit)
{
// wait for fReady:
unique_lock<mutex> lck(mtx);
fThreadsOk = true;
cv.wait(lck); // unlocks lck, and waits for cv be signalled; then
autom. reacquires lck
if (fQuit) break;
if (!fReady) continue;
// work, ie. process the job:
cout << "thread " << id << " has grabbed the job\n";
//...
// notify parent with fReady=false; wakes up also all the other threads
fReady = false;
cv.notify_all();
}
}
int main()
{
fQuit = false;
fReady = false;
fThreadsOk = false;
const size_t N = 10;
vector<thread> threads;
for (size_t i = 0; i < N; ++i)
threads.push_back(thread(threadfunc, i));
// wait so that at least one thread has done "cv.wait(lck)", see above
while (!fThreadsOk) sleep(1);
for (size_t i = 0; i < 10000; ++i)
{
cout << N << " threads ready to race: ";
// notify threads about work to do
{
unique_lock<mutex> lck(mtx);
fReady = true;
cv.notify_all();
// wait till job was done
while (fReady) cv.wait(lck);
}
}
// set fQuit and wake up the threads
{
unique_lock<mutex> lck(mtx);
fQuit = true;
cv.notify_all();
}
// wait till all threads quit:
for (auto& th : threads) th.join();
cout << "pgm finished\n";
return 0;
}
// end of test.cpp
-- System Information:
Debian Release: 8.2
APT prefers stable
APT policy: (500, 'stable')
Architecture: amd64 (x86_64)
Foreign Architectures: i386
Kernel: Linux 4.2.0-0.bpo.1-amd64 (SMP w/4 CPU cores)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: sysvinit (via /sbin/init)
Versions of packages g++-4.9 depends on:
ii gcc-4.9 4.9.2-10
ii gcc-4.9-base 4.9.2-10
ii libc6 2.19-18+deb8u1
ii libcloog-isl4 0.18.2-1+b2
ii libgmp10 2:6.0.0+dfsg-6
ii libisl10 0.12.2-2
ii libmpc3 1.0.2-1
ii libmpfr4 3.1.2-2
ii libstdc++-4.9-dev 4.9.2-10
ii zlib1g 1:1.2.8.dfsg-2+b1
g++-4.9 recommends no packages.
Versions of packages g++-4.9 suggests:
pn g++-4.9-multilib <none>
pn gcc-4.9-doc <none>
pn libstdc++6-4.9-dbg <none>
-- no debconf information
--- End Message ---