Method dereference

src/bthread/execution_queue_inl.h:529–585

}

inline int ExecutionQueueBase::dereference() {
    const uint64_t vref = _versioned_ref.fetch_sub(
            1, butil::memory_order_release);
    const int32_t nref = _ref_of_vref(vref);
    // We need make the fast path as fast as possible, don't put any extra
    // code before this point
    if (nref > 1) {
        return 0;
    }
    const uint64_t id = _this_id;
    if (__builtin_expect(nref == 1, 1)) {
        const uint32_t ver = _version_of_vref(vref);
        const uint32_t id_ver = _version_of_id(id);
        // Besides first successful stop() adds 1 to version, one of
        // those dereferencing nref from 1->0 adds another 1 to version.
        // Notice "one of those": The wait-free address() may make ref of a
        // version-unmatched slot change from 1 to 0 for mutiple times, we
        // have to use version as a guard variable to prevent returning the
        // executor to pool more than once.
        if (__builtin_expect(ver == id_ver || ver == id_ver + 1, 1)) {
            // sees nref:1->0, try to set version=id_ver+2,--nref.
            // No retry: if version changes, the slot is already returned by
            // another one who sees nref:1->0 concurrently; if nref changes,
            // which must be non-zero, the slot will be returned when
            // nref changes from 1->0 again.
            // Example:
            //       stop():  --nref, sees nref:1->0           (1)
            //                try to set version=id_ver+2      (2)
            //    address():  ++nref, unmatched version        (3)
            //                --nref, sees nref:1->0           (4)
            //                try to set version=id_ver+2      (5)
            // 1,2,3,4,5 or 1,3,4,2,5:
            //            stop() succeeds, address() fails at (5).
            // 1,3,2,4,5: stop() fails with (2), the slot will be
            //            returned by (5) of address()
            // 1,3,4,5,2: stop() fails with (2), the slot is already
            //            returned by (5) of address().
            uint64_t expected_vref = vref - 1;
            if (_versioned_ref.compare_exchange_strong(
                        expected_vref, _make_vref(id_ver + 2, 0),
                        butil::memory_order_acquire,
                        butil::memory_order_relaxed)) {
                _on_recycle();
                // We don't return m immediately when the reference count
                // reaches 0 as there might be in processing tasks. Instead
                // _on_recycle would push a `stop_task' after which is executed
                // m would be finally returned and reset
                return 1;
            }
            return 0;
        }
        LOG(FATAL) << "Invalid id=" << id;
        return -1;
    }
    LOG(FATAL) << "Over dereferenced id=" << id;
    return -1;
}