TSAN thread race error detected with Boost intrusive_ptr use - c++

Question

Below code generates a TSAN error(race condition).
Is this a valid error ? or a false positive ?
Object is destroyed only after ref count becomes zero(after all other thread memory operations are visible - with atomic_thread_fence)
If I use a std::shared_ptr instead of boost::intrusive_ptr, then TSAN error disappears.
Since both threads use the object as read-only, I presume this should be safe.
If this is a valid error how do I fix it ?
gcc version - 7.3.1 boost version - 1.72.0
compile command : "g++ -ggdb -I /usr/local/boost_1_72_0 -O3 -fsanitize=thread TSan_Intr_Ptr.cpp -lpthread
"
#include <boost/smart_ptr/intrusive_ptr.hpp>
#include <boost/smart_ptr/detail/spinlock.hpp>
#include <boost/atomic.hpp>
#include <thread>
#include <iostream>
#include <vector>
#include <atomic>
#include <unistd.h>
using namespace std;
struct Shared
{
mutable boost::atomic<int> refcount_;
//From https://www.boost.org/doc/libs/1_72_0/doc/html/atomic/usage_examples.html
friend void intrusive_ptr_add_ref(const Shared * x)
{
x->refcount_.fetch_add(1, boost::memory_order_relaxed);
}
friend void intrusive_ptr_release(const Shared* x)
{
if (x->refcount_.fetch_sub(1, boost::memory_order_release) == 1)
{
boost::atomic_thread_fence(boost::memory_order_acquire);
delete x;
}
}
};
vector<boost::intrusive_ptr<Shared const>> g_vec;
boost::detail::spinlock g_lock = BOOST_DETAIL_SPINLOCK_INIT;
void consumer()
{
while(true)
{
g_lock.lock();
g_vec.clear();
g_lock.unlock();
usleep(10);
}
}
int main()
{
thread thd(consumer);
while(true)
{
boost::intrusive_ptr<Shared const> p(new Shared);
g_lock.lock();
g_vec.push_back(p);
g_lock.unlock();
usleep(1);
}
return 0;
}
TSAN Error
WARNING: ThreadSanitizer: data race (pid=14513)
Write of size 8 at 0x7b0400000010 by main thread:
#0 operator delete(void*) <null> (libtsan.so.0+0x00000006fae4)
#1 intrusive_ptr_release(Shared const*) /Test/TSan_Intr_Ptr_Min.cpp:25 (a.out+0x000000401195)
#2 boost::intrusive_ptr<Shared const>::~intrusive_ptr() /boost_1_72_0/boost/smart_ptr/intrusive_ptr.hpp:98 (a.out+0x000000401195)
#3 main /x01/exch/Test/TSan_Intr_Ptr_Min.cpp:51 (a.out+0x000000401195)
Previous atomic write of size 4 at 0x7b0400000010 by thread T1:
#0 __tsan_atomic32_fetch_sub <null> (libtsan.so.0+0x00000006576f)
#1 boost::atomics::detail::gcc_atomic_operations<4ul, true>::fetch_sub(unsigned int volatile&, unsigned int, boost::memory_order) /boost_1_72_0/boost/atomic/detail/ops_gcc_atomic.hpp:116 (a.out+0x000000401481)
#2 boost::atomics::detail::base_atomic<int, int>::fetch_sub(int, boost::memory_order) volatile /usr/local/boost_1_72_0/boost/atomic/detail/atomic_template.hpp:348 (a.out+0x000000401481)
#3 intrusive_ptr_release(Shared const*) /Test/TSan_Intr_Ptr_Min.cpp:22 (a.out+0x000000401481)
...

Answer 1

it seems using memory_order_acq_rel resolves the issue. (May be https://www.boost.org/doc/libs/1_72_0/doc/html/atomic/usage_examples.html example is in-correct)
friend void intrusive_ptr_add_ref(const Shared * x)
{
x->refcount_.fetch_add(1, boost::memory_order_acq_rel);
}
friend void intrusive_ptr_release(const Shared* x)
{
if (x->refcount_.fetch_sub(1, boost::memory_order_acq_rel) == 1)
{
delete x;
}
}