diff --git a/.gitignore b/.gitignore
index 391be7f9..cc5365a2 100644
--- a/.gitignore
+++ b/.gitignore
@@ -69,4 +69,5 @@ TAGS
 ###########################
 src/rtorrent
 test/rtorrent_Test*
+test/test-suite.log.tmp
 test-driver
diff --git a/src/command_download.cc b/src/command_download.cc
index 0e64bdbc..a8bc128f 100644
--- a/src/command_download.cc
+++ b/src/command_download.cc
@@ -18,6 +18,7 @@
 #include <torrent/net/types.h>
 #include <torrent/peer/connection_list.h>
 #include <torrent/peer/peer_list.h>
+#include <torrent/system/callbacks.h>
 #include <torrent/utils/file_stat.h>
 #include <torrent/utils/log.h>
 #include <torrent/utils/option_strings.h>
@@ -305,8 +306,10 @@ apply_d_add_peer(core::Download* download, const std::string& arg) {
 
   assert(std::this_thread::get_id() == torrent::main_thread::thread_id());
 
+  auto callback_id = torrent::system::make_callback_id();
+
   // Currently discarding SOCK_STREAM.
-  torrent::this_thread::resolver()->resolve_preferred(NULL, host, AF_UNSPEC, AF_INET, [download, port](torrent::c_sa_shared_ptr sa, int err) {
+  torrent::this_thread::resolver()->resolve_preferred(callback_id, host, AF_UNSPEC, AF_INET, [download, port](torrent::c_sa_shared_ptr sa, int err) {
       if (sa == nullptr) {
         lt_log_print(torrent::LOG_TORRENT_WARN, "could not resolve hostname for added peer: %s", gai_strerror(err));
         return;
diff --git a/src/command_tracker.cc b/src/command_tracker.cc
index f61b47fe..5543715f 100644
--- a/src/command_tracker.cc
+++ b/src/command_tracker.cc
@@ -6,6 +6,7 @@
 #include <torrent/net/resolver.h>
 #include <torrent/runtime/network_config.h>
 #include <torrent/runtime/network_manager.h>
+#include <torrent/system/callbacks.h>
 #include <torrent/tracker/dht_controller.h>
 #include <torrent/tracker/tracker.h>
 #include <torrent/utils/log.h>
@@ -48,8 +49,10 @@ apply_dht_add_node(const std::string& arg) {
 
   // TODO: Move this lookup to DhtController.
 
+  auto callback_id = torrent::system::make_callback_id();
+
   // Currently discarding SOCK_STREAM.
-  torrent::this_thread::resolver()->resolve_specific(nullptr, host_str, PF_INET, [host_str, port](torrent::c_sa_shared_ptr sa, int err) {
+  torrent::this_thread::resolver()->resolve_specific(callback_id, host_str, PF_INET, [host_str, port](torrent::c_sa_shared_ptr sa, int err) {
       if (sa == nullptr) {
         lt_log_print(torrent::LOG_DHT_ERROR, "dht.add_node : could not resolve host : %s (%s)", gai_strerror(err), host_str.c_str());
         return;
diff --git a/src/globals.h b/src/globals.h
index dca7ae58..7d5cc9e5 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -25,13 +25,11 @@ namespace scgi_thread {
 torrent::system::Thread* thread();
 std::thread::id          thread_id();
 
-void                    callback(void* target, std::function<void ()>&& fn);
-void                    cancel_callback(void* target);
-void                    cancel_callback_and_wait(void* target);
+void                     callback_interrupt(torrent::system::callback_id& id, std::function<void ()>&& fn);
 
-rpc::SCgi*              scgi();
-void                    set_scgi(rpc::SCgi* scgi);
-void                    set_rpc_log(const std::string& filename);
+rpc::SCgi*               scgi();
+void                     set_scgi(rpc::SCgi* scgi);
+void                     set_rpc_log(const std::string& filename);
 
 } // namespace torrent::scgi_thread
 
@@ -41,9 +39,9 @@ namespace session_thread {
 torrent::system::Thread* thread();
 std::thread::id          thread_id();
 
-void                    callback(void* target, std::function<void ()>&& fn);
-void                    cancel_callback(void* target);
-void                    cancel_callback_and_wait(void* target);
+void                     callback(std::function<void ()>&& fn);
+void                     callback(torrent::system::callback_id& id, std::function<void ()>&& fn);
+void                     cancel_callback(torrent::system::callback_id& id);
 
 session::SessionManager* manager();
 std::string              session_path();
diff --git a/src/rpc/scgi_task.cc b/src/rpc/scgi_task.cc
index aa93a8c7..0af85c65 100644
--- a/src/rpc/scgi_task.cc
+++ b/src/rpc/scgi_task.cc
@@ -14,7 +14,7 @@
 #include <torrent/net/poll.h>
 #include <torrent/runtime/socket_manager.h>
 #include <torrent/utils/log.h>
-#include <torrent/system/thread.h>
+#include <torrent/system/callbacks.h>
 
 #include "control.h"
 #include "globals.h"
@@ -24,6 +24,12 @@
 
 namespace rpc {
 
+SCgiTask::SCgiTask()
+  : m_callback_id(torrent::system::make_callback_id()) {
+
+  set_file_descriptor(-1);
+}
+
 void
 SCgiTask::open(SCgi* parent, int fd) {
   set_file_descriptor(fd);
@@ -67,8 +73,7 @@ SCgiTask::close() {
   if (!is_open())
     return;
 
-  torrent::main_thread::thread()->cancel_callback_and_wait(this);
-  torrent::system::Thread::self()->cancel_callback(this);
+  torrent::system::cancel_callback_and_wait(m_callback_id, scgi_thread::thread(), torrent::main_thread::thread());
 
   torrent::runtime::socket_manager()->close_event_or_throw(this, [this]() {
       torrent::this_thread::poll()->remove_and_close(this);
@@ -86,13 +91,13 @@ SCgiTask::close() {
 void
 SCgiTask::event_read() {
   int read_length = m_buffer.size() - m_position;
-  
+
   if (m_content_length == 0)
     read_length--;
 
   if (read_length <= 0)
     throw torrent::internal_error("SCgiTask::event_read() no space in buffer for event_read.");
-  
+
   int bytes = ::recv(m_fileDesc, m_buffer.data() + m_position, read_length, 0);
 
   if (bytes <= 0) {
@@ -321,7 +326,7 @@ SCgiTask::detect_content_type(const std::string& content_type) {
 
 void
 SCgiTask::receive_call(const char* buffer, uint32_t length) {
-  assert(torrent::system::Thread::self() == scgi_thread::thread());
+  assert(torrent::this_thread::thread() == scgi_thread::thread());
 
   RpcManager::RPCType rpc_type;
 
@@ -348,7 +353,7 @@ SCgiTask::receive_call(const char* buffer, uint32_t length) {
       m_result_mutex.lock();
       m_result_mutex.unlock();
 
-      scgi_thread::thread()->callback_interrupt_polling(this, [this]() {
+      scgi_thread::callback_interrupt(m_callback_id, [this]() {
           if (!is_open())
             return;
 
@@ -368,7 +373,7 @@ SCgiTask::receive_call(const char* buffer, uint32_t length) {
   m_result_mutex.lock();
   m_result_mutex.unlock();
 
-  torrent::main_thread::thread()->callback_interrupt_polling(this, [this, rpc_type, buffer, length, result_callback]() {
+  torrent::main_thread::callback_interrupt(m_callback_id, [this, rpc_type, buffer, length, result_callback]() {
       // Memory barrier for the input data.
       // std::atomic_thread_fence(std::memory_order_acquire);
       m_result_mutex.lock();
@@ -383,7 +388,7 @@ SCgiTask::receive_call(const char* buffer, uint32_t length) {
 
 void
 SCgiTask::receive_write(const char* buffer, uint32_t length) {
-  assert(torrent::system::Thread::self() == torrent::main_thread::thread());
+  assert(torrent::this_thread::thread() == torrent::main_thread::thread());
 
   if (buffer == nullptr || length > (100 << 20))
     throw torrent::internal_error("SCgiTask::receive_write(...) received bad input.");
diff --git a/src/rpc/scgi_task.h b/src/rpc/scgi_task.h
index e012e9be..1b3961b1 100644
--- a/src/rpc/scgi_task.h
+++ b/src/rpc/scgi_task.h
@@ -18,7 +18,7 @@ public:
 
   enum ContentType { XML, JSON };
 
-  SCgiTask() { m_fileDesc = -1; }
+  SCgiTask();
 
   const char*         type_name() const override { return "scgi-task"; }
 
@@ -54,7 +54,8 @@ private:
   void                plaintext_response(const char* buffer, uint32_t content_length);
   void                gzip_response(const char* buffer, uint32_t content_length);
 
-  SCgi*               m_parent{};
+  SCgi*                        m_parent{};
+  torrent::system::callback_id m_callback_id;
 
   std::mutex          m_result_mutex;
 
diff --git a/src/scgi/thread_scgi.cc b/src/scgi/thread_scgi.cc
index ada7b7ad..3ff0d5f6 100644
--- a/src/scgi/thread_scgi.cc
+++ b/src/scgi/thread_scgi.cc
@@ -12,11 +12,6 @@
 
 namespace scgi {
 
-class ThreadScgiInternal {
-public:
-  static ThreadScgi* thread_scgi() { return ThreadScgi::internal_thread_scgi(); }
-};
-
 ThreadScgi* ThreadScgi::m_thread_scgi{};
 
 void
@@ -60,7 +55,7 @@ ThreadScgi::set_scgi(rpc::SCgi* scgi) {
 
   change_rpc_log();
 
-  callback(nullptr, [this]() {
+  callback([this]() {
       if (m_scgi == nullptr)
         throw torrent::internal_error("Tried to start SCGI but object was not present.");
 
@@ -72,7 +67,7 @@ ThreadScgi::set_scgi(rpc::SCgi* scgi) {
 
 void
 ThreadScgi::set_rpc_log(const std::string& filename) {
-  callback(nullptr, [this, filename]() {
+  callback([this, filename]() {
       m_rpc_log_filename = filename;
       change_rpc_log();
     });
@@ -127,15 +122,13 @@ ThreadScgi::next_timeout() {
 
 namespace scgi_thread {
 
-torrent::system::Thread* thread()                         { return scgi::ThreadScgiInternal::thread_scgi(); }
-std::thread::id          thread_id()                      { return scgi::ThreadScgiInternal::thread_scgi()->thread_id(); }
+torrent::system::Thread* thread()                         { return scgi::ThreadScgi::thread_scgi(); }
+std::thread::id          thread_id()                      { return scgi::ThreadScgi::thread_scgi()->thread_id(); }
 
-void callback(void* target, std::function<void ()>&& fn) { scgi::ThreadScgiInternal::thread_scgi()->callback(target, std::move(fn)); }
-void cancel_callback(void* target)                       { scgi::ThreadScgiInternal::thread_scgi()->cancel_callback(target); }
-void cancel_callback_and_wait(void* target)              { scgi::ThreadScgiInternal::thread_scgi()->cancel_callback_and_wait(target); }
+void        callback_interrupt(torrent::system::callback_id& id, std::function<void ()>&& fn) { scgi::ThreadScgi::thread_scgi()->callback_interrupt(id, std::move(fn)); }
 
-rpc::SCgi*  scgi()                                       { return scgi::ThreadScgiInternal::thread_scgi()->scgi(); }
-void        set_scgi(rpc::SCgi* scgi)                    { scgi::ThreadScgiInternal::thread_scgi()->set_scgi(scgi); }
-void        set_rpc_log(const std::string& filename)     { scgi::ThreadScgiInternal::thread_scgi()->set_rpc_log(filename); }
+rpc::SCgi*  scgi()                                       { return scgi::ThreadScgi::thread_scgi()->scgi(); }
+void        set_scgi(rpc::SCgi* scgi)                    { scgi::ThreadScgi::thread_scgi()->set_scgi(scgi); }
+void        set_rpc_log(const std::string& filename)     { scgi::ThreadScgi::thread_scgi()->set_rpc_log(filename); }
 
 } // namespace scgi_thread
diff --git a/src/scgi/thread_scgi.h b/src/scgi/thread_scgi.h
index 824ffa98..38f5db27 100644
--- a/src/scgi/thread_scgi.h
+++ b/src/scgi/thread_scgi.h
@@ -11,8 +11,6 @@ class SCgi;
 
 namespace scgi {
 
-class ThreadScgiInternal;
-
 class ThreadScgi : public torrent::system::Thread {
 public:
 
@@ -28,8 +26,6 @@ public:
   void                set_rpc_log(const std::string& filename);
 
 protected:
-  friend class ThreadScgiInternal;
-
   ThreadScgi() = default;
 
   static auto         internal_thread_scgi() { return m_thread_scgi; }
@@ -43,9 +39,9 @@ private:
   void                task_touch_log();
   void                change_rpc_log();
 
-  static ThreadScgi*  m_thread_scgi;
+  static ThreadScgi*      m_thread_scgi;
 
-  std::atomic<rpc::SCgi*> m_scgi{nullptr};
+  std::atomic<rpc::SCgi*> m_scgi{};
   std::string             m_rpc_log_filename;
 };
 
diff --git a/src/session/session_manager.cc b/src/session/session_manager.cc
index 3d2468be..933ad5fa 100644
--- a/src/session/session_manager.cc
+++ b/src/session/session_manager.cc
@@ -5,6 +5,7 @@
 #include <cassert>
 #include <cstdlib>
 #include <torrent/exceptions.h>
+#include <torrent/system/callbacks.h>
 #include <torrent/utils/log.h>
 
 #include "globals.h"
@@ -23,6 +24,7 @@ namespace session {
 
 SessionManager::SessionManager(torrent::system::Thread* thread)
   : m_thread(thread),
+    m_callback_id(torrent::system::make_callback_id()),
     m_lockfile(std::make_unique<utils::Lockfile>()) {
 }
 
@@ -221,8 +223,7 @@ SessionManager::cleanup() {
 
   LT_LOG("session manager cleaned up", 0);
 
-  session_thread::cancel_callback(this);
-  torrent::main_thread::cancel_callback(this);
+  torrent::system::cancel_callback_and_wait(m_callback_id, session_thread::thread(), torrent::main_thread::thread());
 }
 
 void
@@ -233,7 +234,9 @@ SessionManager::callback_pending_builds() {
   if (m_callback_scheduled_process_pending_builds.exchange(true))
     return;
 
-  torrent::main_thread::callback(this, [this]() { process_pending_builds(false); });
+  torrent::main_thread::callback(m_callback_id, [this]() {
+      process_pending_builds(false);
+    });
 }
 
 void
@@ -247,7 +250,9 @@ SessionManager::callback_save_request() {
   if (m_callback_scheduled_process_saves_request.exchange(true))
     return;
 
-  session_thread::callback(this, [this]() { process_save_request(); });
+  session_thread::callback(m_callback_id, [this]() {
+      process_save_request();
+    });
 }
 
 void
@@ -255,7 +260,9 @@ SessionManager::callback_finished_saves() {
   if (m_callback_scheduled_process_finished_saves.exchange(true))
     return;
 
-  session_thread::callback(this, [this]() { process_finished_saves(); });
+  session_thread::callback(m_callback_id, [this]() {
+      process_finished_saves();
+    });
 }
 
 void
diff --git a/src/session/session_manager.h b/src/session/session_manager.h
index 4fa76843..eff135ac 100644
--- a/src/session/session_manager.h
+++ b/src/session/session_manager.h
@@ -89,37 +89,39 @@ private:
   bool                replace_save_request_unsafe(SaveRequest& download);
   bool                remove_completely_unsafe(core::Download* download, std::unique_lock<std::mutex>& lock);
 
-  torrent::system::Thread* m_thread;
+  using ProcessingSave = std::pair<std::future<void>, SaveRequest>;
 
-  bool                m_freeze_info{};
-  std::string         m_path;
-  bool                m_use_fsyncdisk{true};
-  bool                m_use_lock{true};
+  torrent::system::Thread*     m_thread;
+  torrent::system::callback_id m_callback_id;
 
-  std::mutex          m_mutex;
-  bool                m_active{};
+  bool                         m_freeze_info{};
+  std::string                  m_path;
+  bool                         m_use_fsyncdisk{true};
+  bool                         m_use_lock{true};
 
-  typedef std::pair<std::future<void>, SaveRequest> ProcessingSave;
+  align_cacheline std::mutex   m_mutex;
 
-  std::deque<SaveRequest>     m_save_requests;
-  std::atomic<size_t>         m_save_request_counter{};
-  std::list<ProcessingSave>   m_processing_saves;
-  std::atomic<size_t>         m_processing_save_counter{};
-  std::condition_variable     m_finished_condition;
-  std::vector<ProcessingSave> m_finished_saves;
+  bool                         m_active{};
 
-  std::atomic<bool>           m_callback_scheduled_process_pending_builds{};
-  std::atomic<bool>           m_callback_scheduled_process_saves_request{};
-  std::atomic<bool>           m_callback_scheduled_process_finished_saves{};
+  std::deque<SaveRequest>      m_save_requests;
+  std::atomic<size_t>          m_save_request_counter{};
+  std::list<ProcessingSave>    m_processing_saves;
+  std::atomic<size_t>          m_processing_save_counter{};
+  std::condition_variable      m_finished_condition;
+  std::vector<ProcessingSave>  m_finished_saves;
+
+  std::atomic<bool>            m_callback_scheduled_process_pending_builds{};
+  std::atomic<bool>            m_callback_scheduled_process_saves_request{};
+  std::atomic<bool>            m_callback_scheduled_process_finished_saves{};
 
   std::unique_ptr<utils::Lockfile> m_lockfile;
 
-  std::chrono::microseconds   m_last_storage_error_message{};
-  unsigned int                m_ignored_storage_error_count{};
+  std::chrono::microseconds    m_last_storage_error_message{};
+  unsigned int                 m_ignored_storage_error_count{};
 
   // Pending builds are only ever locked by main thread.
-  std::mutex                  m_pending_builds_mutex;
-  std::deque<core::Download*> m_pending_builds;
+  std::mutex                   m_pending_builds_mutex;
+  std::deque<core::Download*>  m_pending_builds;
 };
 
 inline bool        SessionManager::is_used() const            { return !m_path.empty(); }
diff --git a/src/session/thread_session.cc b/src/session/thread_session.cc
index bf698beb..e9015cc9 100644
--- a/src/session/thread_session.cc
+++ b/src/session/thread_session.cc
@@ -4,15 +4,11 @@
 
 #include <torrent/exceptions.h>
 
+#include "globals.h"
 #include "session/session_manager.h"
 
 namespace session {
 
-class ThreadSessionInternal {
-public:
-  static ThreadSession* thread_session() { return ThreadSession::internal_thread_session(); }
-};
-
 ThreadSession* ThreadSession::m_thread_session{nullptr};
 
 void
@@ -74,14 +70,14 @@ ThreadSession::next_timeout() {
 
 namespace session_thread {
 
-torrent::system::Thread* thread()                         { return session::ThreadSessionInternal::thread_session(); }
-std::thread::id          thread_id()                      { return session::ThreadSessionInternal::thread_session()->thread_id(); }
+torrent::system::Thread* thread()                                           { return session::ThreadSession::thread_session(); }
+std::thread::id          thread_id()                                        { return thread()->thread_id(); }
 
-void callback(void* target, std::function<void ()>&& fn) { session::ThreadSessionInternal::thread_session()->callback(target, std::move(fn)); }
-void cancel_callback(void* target)                       { session::ThreadSessionInternal::thread_session()->cancel_callback(target); }
-void cancel_callback_and_wait(void* target)              { session::ThreadSessionInternal::thread_session()->cancel_callback_and_wait(target); }
+void callback(std::function<void()>&& fn)                                   { thread()->callback(std::move(fn)); }
+void callback(torrent::system::callback_id& id, std::function<void()>&& fn) { thread()->callback(id, std::move(fn)); }
+void cancel_callback(torrent::system::callback_id& id)                      { thread()->cancel_callback(id); }
 
-session::SessionManager* manager()                       { return session::ThreadSessionInternal::thread_session()->manager(); }
-std::string              session_path()                  { return session::ThreadSessionInternal::thread_session()->manager()->path(); }
+session::SessionManager* manager()                                          { return session::ThreadSession::thread_session()->manager(); }
+std::string              session_path()                                     { return manager()->path(); }
 
 } // namespace session_thread