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[Daniel van Vugt <daniel.van.vugt@xxxxxxxxxxxxx>]

** Also affects: mir (Ubuntu)
   Importance: Undecided
       Status: New

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https://bugs.launchpad.net/bugs/1464690

Title:
  [regression] titlebar in "canonical" example WM + KeyRepeatDispatcher
  causes deadlock

Status in Mir:
  Fix Committed
Status in mir package in Ubuntu:
  New

Bug description:
  Introducing the KeyRepeatDispatcher means that input events can now
  occur on two threads: the "Input dispatch" thread and the GLibMainLoop
  thread. This leads to lock contention in EventFilterChainDispatcher
  which isn't necessarily fatal - but I think we'd be better using a
  single thread for dispatching input (to impose an order on events).

  What is fatal is that this breaks the (dubious) assumption in
  CanonicalWindowManagerPolicyCopy that we can wait for swap_buffers()
  while handling an input event. This leads to a deadlock if an input
  event is on the GLibMainLoop thread (in the original description this
  is in EventFilterChainDispatcher, but that just prevents it occurring
  in BasicWindowManagerCopy).

  Original text:

  I caused this while testing a branch (lp:~alan-
  griffiths/mir/msh_SystemCompositorWindowManager), but I don't think
  the problem originates there:

  $ sudo bin/mir_demo_server --file /tmp/mir_socket --vt 2 --arw-file
  --window-manager system-compositor

  $ bin/mir_demo_server --host /tmp/mir_socket --window-manager
  canonical

  $ bin/mir_demo_client_multiwin & bin/mir_demo_client_eglplasma &

  After a few Alt+Tab focus switches things lock up in the nested
  server. Attaching with GDB gets the following stack traces:

  The event loop (via) KeyRepeatDispatcher is waiting for a lock in
  EventFilterChainDispatcher::handle().

  #0  __lll_lock_wait () at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:135
  #1  0x00007ffff6960cfd in __GI___pthread_mutex_lock (mutex=0x7ca060) at ../nptl/pthread_mutex_lock.c:80
  #2  0x00007ffff76e7815 in __gthread_mutex_lock (__mutex=0x7ca060)
      at /usr/include/x86_64-linux-gnu/c++/4.9/bits/gthr-default.h:748
  #3  0x00007ffff76e8738 in std::mutex::lock (this=0x7ca060) at /usr/include/c++/4.9/mutex:135
  #4  0x00007ffff76e8b2e in std::lock_guard<std::mutex>::lock_guard (this=0x7fffffffdbb0, __m=...)
      at /usr/include/c++/4.9/mutex:377
  #5  0x00007ffff775d1db in mir::input::EventFilterChainDispatcher::handle (this=0x7ca050, event=...)
      at /home/alan/display_server/mir3/src/server/input/event_filter_chain_dispatcher.cpp:34
  #6  0x00007ffff775d510 in mir::input::EventFilterChainDispatcher::dispatch (this=0x7ca050, event=...)
      at /home/alan/display_server/mir3/src/server/input/event_filter_chain_dispatcher.cpp:67
  #7  0x00007ffff7796e2e in mir::input::KeyRepeatDispatcher::<lambda()>::operator()(void) (__closure=0x7fffe000a0a0)
      at /home/alan/display_server/mir3/src/server/input/key_repeat_dispatcher.cpp:117
  #8  0x00007ffff7797512 in std::_Function_handler<void(), mir::input::KeyRepeatDispatcher::handle_key_input(MirInputDeviceId, const MirKeyboardEvent*)::<lambda()> >::_M_invoke(const std::_Any_data &) (__functor=...) at /usr/include/c++/4.9/functional:2039
  #9  0x00007ffff76ee54c in std::function<void ()>::operator()() const (this=0x7fffe0000e68)
      at /usr/include/c++/4.9/functional:2439
  #10 0x00007ffff773b626 in mir::BasicCallback::operator() (this=0x7fffe0000e60)
      at /home/alan/display_server/mir3/src/server/basic_callback.cpp:28
  #11 0x00007ffff773b111 in mir::LockableCallbackWrapper::operator() (this=0x7fffe00036f0)
      at /home/alan/display_server/mir3/src/server/lockable_callback_wrapper.cpp:41
  #12 0x00007ffff770face in mir::detail::TimerGSource::dispatch (source=0x949320)
      at /home/alan/display_server/mir3/src/server/glib_main_loop_sources.cpp:299
  #13 0x00007ffff5443c3d in g_main_context_dispatch () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
  #14 0x00007ffff5443f20 in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
  #15 0x00007ffff5443fcc in g_main_context_iteration () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
  #16 0x00007ffff7705f91 in mir::GLibMainLoop::run (this=0x6f6e70)
  ---Type <return> to continue, or q <return> to quit---

  The lock is held by the window manager (which is trying to paint a
  titlebar).

  #0  pthread_cond_wait@@GLIBC_2.3.2 () at ../sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:185
  #1  0x00007ffff64f7d1c in std::condition_variable::wait(std::unique_lock<std::mutex>&) ()
     from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
  #2  0x00000000004b270b in std::condition_variable::wait<(anonymous namespace)::swap_buffers(const std::shared_ptr<mir::scene::Surface>&, mir::graphics::Buffer*&)::<lambda()> >(std::unique_lock<std::mutex> &, (anonymous namespace)::<lambda()>) (
      this=0x7fffee2f21b0, __lock=..., __p=...) at /usr/include/c++/4.9/condition_variable:98
  #3  0x00000000004aeaa4 in (anonymous namespace)::swap_buffers (surface=std::shared_ptr (count 4, weak 4) 0x7fffc4006750,
      surface_buffer=@0x7fffc4006fc0: 0x7fffc40113f0)
      at /home/alan/display_server/mir3/examples/server_example_canonical_window_manager.cpp:277
  #4  0x00000000004aecea in (anonymous namespace)::paint_titlebar (titlebar=std::shared_ptr (count 4, weak 4) 0x7fffc4006750,
      titlebar_info=..., intensity=255) at /home/alan/display_server/mir3/examples/server_example_canonical_window_manager.cpp:297
  #5  0x00000000004b0f45 in mir::examples::CanonicalWindowManagerPolicyCopy::select_active_surface (this=0x944cd8,
      surface=std::shared_ptr (count 3, weak 5) 0x7fffc4004fd0)
      at /home/alan/display_server/mir3/examples/server_example_canonical_window_manager.cpp:742
  #6  0x00000000004b042a in mir::examples::CanonicalWindowManagerPolicyCopy::handle_keyboard_event (this=0x944cd8,
      event=0x7fffe0000910) at /home/alan/display_server/mir3/examples/server_example_canonical_window_manager.cpp:577
  #7  0x00000000004a0c5e in mir::examples::BasicWindowManagerCopy<mir::examples::CanonicalWindowManagerPolicyCopy, mir::examples::CanonicalSessionInfoCopy, mir::examples::CanonicalSurfaceInfoCopy>::handle_keyboard_event (this=0x944cc0, event=0x7fffe0000910)
      at /home/alan/display_server/mir3/examples/server_example_basic_window_manager.h:190
  #8  0x00007ffff78beccb in mir::shell::AbstractShell::handle (this=0x944bf0, event=...)
      at /home/alan/display_server/mir3/src/server/shell/abstract_shell.cpp:245
  #9  0x00007ffff775d27c in mir::input::EventFilterChainDispatcher::handle (this=0x7ca050, event=...)
      at /home/alan/display_server/mir3/src/server/input/event_filter_chain_dispatcher.cpp:45
  #10 0x00007ffff775d510 in mir::input::EventFilterChainDispatcher::dispatch (this=0x7ca050, event=...)
      at /home/alan/display_server/mir3/src/server/input/event_filter_chain_dispatcher.cpp:67
  #11 0x00007ffff7796d22 in mir::input::KeyRepeatDispatcher::dispatch (this=0x7ca0d0, event=...)
      at /home/alan/display_server/mir3/src/server/input/key_repeat_dispatcher.cpp:63
  ---Type <return> to continue, or q <return> to quit---

  I guess the titlebar paint doesn't complete because the event loop is
  waiting.

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