[Octave-bug-tracker] [bug #58678] Deadlock when deleting a figure while another figure is beingdeleted

[Pantxo Diribarne]

URL:
  <https://savannah.gnu.org/bugs/?58678>

                 Summary: Deadlock when deleting a figure while another figure
is beingdeleted
                 Project: GNU Octave
            Submitted by: pantxo
            Submitted on: lun. 29 juin 2020 20:27:19 UTC
                Category: Plotting with OpenGL
                Severity: 4 - Important
                Priority: 5 - Normal
              Item Group: Segfault, Bus Error, etc.
                  Status: Confirmed
             Assigned to: None
         Originator Name: 
        Originator Email: 
             Open/Closed: Open
                 Release: dev
         Discussion Lock: Any
        Operating System: GNU/Linux

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Details:

Here is a robust way to deadlock with Octave 5.1, stable and default:


hf = figure ();
hf2 = figure ();
addlistener (hf, "beingdeleted", @() delete (hf2));
delete (hf);


I dug into this and it turns out that this sequence provokes a recursive call
to F__go_delete__. This function uses an octave::autolock to lock the graphics
mutex and then unlock it at return. When graphics_toolkit::finalize is called,
the graphics mutex has been locked twice and using gh_manager::unlock is not
enough to unlock it (unlocking twice avoids the deadlock).

IOW, I just discovered that octave_mutex is a recursive mutex, and there is no
way to unlock it unless you know the number of times it has been previously
locked in the same thread. See also [1] for a discussion about recursive
mutexes.

So both qt_graphics_toolkit::initialize and finalize, can possibly deadlock. 

PS: Contrary to what I have said in [2], our implementation of mutex is thus
very well suited for recursive use with octave::autolock.

PPS: I tagged this bug as segfault since a deadlock looks as critical.
 
[1] http://www.zaval.org/resources/library/butenhof1.html
[2]
https://octave.1599824.n4.nabble.com/segfaults-in-graphics-code-likely-threading-issues-td4697518.html




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