A little simplification of the Perl side threading:

Only single level Perl worker threads are allowed.

lib/Slic3r.pm

@@ -91,11 +91,9 @@ use constant LOOP_CLIPPING_LENGTH_OVER_NOZZLE_DIAMETER => 0.15;
 # use constant SCALED_RESOLUTION      => RESOLUTION / SCALING_FACTOR;
 use constant INFILL_OVERLAP_OVER_SPACING  => 0.3;
 
-# Keep track of threads we created. Each thread keeps its own list of threads it spwaned.
-my @my_threads = ();
-my @threads : shared = ();
+# Keep track of threads we created. Perl worker threads shall not create further threads.
+my @threads = ();
 my $pause_sema = Thread::Semaphore->new;
-my $parallel_sema;
 my $paused = 0;
 
 # Set the logging level at the Slic3r XS module.
@@ -104,19 +102,11 @@ set_logging_level($Slic3r::loglevel);
 
 sub spawn_thread {
     my ($cb) = @_;
-    
-    my $parent_tid = threads->tid;
-    lock @threads;
-    
     @_ = ();
     my $thread = threads->create(sub {
-        @my_threads = ();
-        
-        Slic3r::debugf "Starting thread %d (parent: %d)...\n", threads->tid, $parent_tid;
+        Slic3r::debugf "Starting thread %d...\n", threads->tid;
         local $SIG{'KILL'} = sub {
             Slic3r::debugf "Exiting thread %d...\n", threads->tid;
-            $parallel_sema->up if $parallel_sema;
-            kill_all_threads();
             Slic3r::thread_cleanup();
             threads->exit();
         };
@@ -126,7 +116,6 @@ sub spawn_thread {
         };
         $cb->();
     });
-    push @my_threads, $thread->tid;
     push @threads, $thread->tid;
     return $thread;
 }
@@ -143,13 +132,6 @@ sub spawn_thread {
 # object in a thread, make sure the main thread still holds a
 # reference so that it won't be destroyed in thread.
 sub thread_cleanup {
-    return if !$Slic3r::have_threads;
-    
-    if (threads->tid == 0) {
-        warn "Calling thread_cleanup() from main thread\n";
-        return;
-    }
-
     # prevent destruction of shared objects
     no warnings 'redefine';
     *Slic3r::BridgeDetector::DESTROY        = sub {};
@@ -169,9 +151,6 @@ sub thread_cleanup {
     *Slic3r::ExtrusionPath::Collection::DESTROY = sub {};
     *Slic3r::ExtrusionSimulator::DESTROY    = sub {};
     *Slic3r::Flow::DESTROY                  = sub {};
-# Fillers are only being allocated in worker threads, which are not going to be forked.
-# Therefore the Filler instances shall be released at the end of the thread.
-#    *Slic3r::Filler::DESTROY                = sub {};
     *Slic3r::GCode::DESTROY                 = sub {};
     *Slic3r::GCode::PlaceholderParser::DESTROY = sub {};
     *Slic3r::GCode::Sender::DESTROY         = sub {};
@@ -200,44 +179,37 @@ sub thread_cleanup {
     return undef;  # this prevents a "Scalars leaked" warning
 }
 
-sub get_running_threads {
-    return grep defined($_), map threads->object($_), @_;
+sub _get_running_threads {
+    return grep defined($_), map threads->object($_), @threads;
 }
 
 sub kill_all_threads {
-    # if we're the main thread, we send SIGKILL to all the running threads
-    if (threads->tid == 0) {
-        lock @threads;
-        foreach my $thread (get_running_threads(@threads)) {
-            Slic3r::debugf "Thread %d killing %d...\n", threads->tid, $thread->tid;
-            $thread->kill('KILL');
-        }
-        
-        # unlock semaphore before we block on wait
-        # otherwise we'd get a deadlock if threads were paused
-        resume_all_threads();
+    # Send SIGKILL to all the running threads to let them die.
+    foreach my $thread (_get_running_threads) {
+        Slic3r::debugf "Thread %d killing %d...\n", threads->tid, $thread->tid;
+        $thread->kill('KILL');
     }
-    
+    # unlock semaphore before we block on wait
+    # otherwise we'd get a deadlock if threads were paused
+    resume_all_threads();
     # in any thread we wait for our children
-    foreach my $thread (get_running_threads(@my_threads)) {
+    foreach my $thread (_get_running_threads) {
         Slic3r::debugf "  Thread %d waiting for %d...\n", threads->tid, $thread->tid;
         $thread->join;  # block until threads are killed
         Slic3r::debugf "    Thread %d finished waiting for %d...\n", threads->tid, $thread->tid;
     }
-    @my_threads = ();
+    @threads = ();
 }
 
 sub pause_all_threads {
     return if $paused;
-    lock @threads;
     $paused = 1;
     $pause_sema->down;
-    $_->kill('STOP') for get_running_threads(@threads);
+    $_->kill('STOP') for _get_running_threads;
 }
 
 sub resume_all_threads {
     return unless $paused;
-    lock @threads;
     $paused = 0;
     $pause_sema->up;
 }

lib/Slic3r/GUI.pm

@@ -350,7 +350,6 @@ sub check_version {
         my $response = $ua->get('http://slic3r.org/updatecheck');
         Wx::PostEvent($self, Wx::PlThreadEvent->new(-1, $VERSION_CHECK_EVENT,
             threads::shared::shared_clone([ $response->is_success, $response->decoded_content, $manual_check ])));
-        
         Slic3r::thread_cleanup();
     })->detach;
 }

lib/Slic3r/Print/Object.pm

@@ -14,11 +14,6 @@ use Slic3r::Surface ':types';
 # If enabled, phases of prepare_infill will be written into SVG files to an "out" directory.
 our $SLIC3R_DEBUG_SLICE_PROCESSING = 0;
 
-sub region_volumes {
-    my $self = shift;
-    return [ map $self->get_region_volumes($_), 0..($self->region_count - 1) ];
-}
-
 sub layers {
     my $self = shift;
     return [ map $self->get_layer($_), 0..($self->layer_count - 1) ];