diff --git a/include/core/spinlock.h b/include/core/spinlock.h
new file mode 100644
index 0000000000..6228e5d9ce
--- /dev/null
+++ b/include/core/spinlock.h
@@ -0,0 +1,60 @@
+/*
+* This program source code file is part of KiCad, a free EDA CAD application.
+*
+* Copyright (C) 2021 KiCad Developers, see AUTHORS.txt for contributors.
+*
+* This program is free software: you can redistribute it and/or modify it
+* under the terms of the GNU General Public License as published by the
+* Free Software Foundation, either version 3 of the License, or (at your
+* option) any later version.
+*
+* This program is distributed in the hope that it will be useful, but
+* WITHOUT ANY WARRANTY; without even the implied warranty of
+* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+* General Public License for more details.
+*
+* You should have received a copy of the GNU General Public License along
+* with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+#ifndef __KICAD_SPINLOCK_H
+#define __KICAD_SPINLOCK_H
+
+#include <atomic>
+
+
+/**
+ * A trivial spinlock implementation with no optimization.  Don't use if congestion is expected!
+ */
+class KISPINLOCK
+{
+public:
+    KISPINLOCK() :
+        m_lock( false )
+    {}
+
+    void lock()
+    {
+        while( m_lock.exchange( true, std::memory_order_acquire ) );
+    }
+
+    bool try_lock()
+    {
+        return !m_lock.exchange( true, std::memory_order_acquire );
+    }
+
+    void unlock()
+    {
+        m_lock.store( false, std::memory_order_release );
+    }
+
+    bool test()
+    {
+        return m_lock.load( std::memory_order_acquire );
+    }
+
+private:
+    std::atomic<bool> m_lock;
+};
+
+#endif
diff --git a/pcbnew/connectivity/connectivity_data.h b/pcbnew/connectivity/connectivity_data.h
index 8cc03a8358..a59f21486f 100644
--- a/pcbnew/connectivity/connectivity_data.h
+++ b/pcbnew/connectivity/connectivity_data.h
@@ -28,6 +28,7 @@
 #define __CONNECTIVITY_DATA_H
 
 #include <core/typeinfo.h>
+#include <core/spinlock.h>
 
 #include <memory>
 #include <mutex>
@@ -261,7 +262,7 @@ public:
         return m_connAlgo;
     }
 
-    std::mutex& GetLock()
+    KISPINLOCK& GetLock()
     {
         return m_lock;
     }
@@ -307,7 +308,7 @@ private:
 
     bool m_skipRatsnest = false;
 
-    std::mutex m_lock;
+    KISPINLOCK m_lock;
 
     /// Map of netcode -> netclass the net is a member of; used for ratsnest painting
     std::map<int, wxString> m_netclassMap;
diff --git a/pcbnew/edit_zone_helpers.cpp b/pcbnew/edit_zone_helpers.cpp
index 2465ff6537..482b34fe9b 100644
--- a/pcbnew/edit_zone_helpers.cpp
+++ b/pcbnew/edit_zone_helpers.cpp
@@ -125,6 +125,8 @@ void PCB_EDIT_FRAME::Edit_Zone_Params( ZONE* aZone )
 
     commit.Stage( pickedList );
 
+    std::lock_guard<KISPINLOCK> lock( GetBoard()->GetConnectivity()->GetLock() );
+
     if( zones_to_refill.size() )
     {
         ZONE_FILLER filler( GetBoard(), &commit );
diff --git a/pcbnew/ratsnest/ratsnest_viewitem.cpp b/pcbnew/ratsnest/ratsnest_viewitem.cpp
index 480dc4c3fc..83c4a59bd5 100644
--- a/pcbnew/ratsnest/ratsnest_viewitem.cpp
+++ b/pcbnew/ratsnest/ratsnest_viewitem.cpp
@@ -63,7 +63,7 @@ const BOX2I RATSNEST_VIEWITEM::ViewBBox() const
 
 void RATSNEST_VIEWITEM::ViewDraw( int aLayer, KIGFX::VIEW* aView ) const
 {
-    std::unique_lock<std::mutex> lock( m_data->GetLock(), std::try_to_lock );
+    std::unique_lock<KISPINLOCK> lock( m_data->GetLock(), std::try_to_lock );
 
     if( !lock )
         return;
diff --git a/pcbnew/tools/zone_create_helper.cpp b/pcbnew/tools/zone_create_helper.cpp
index cec1e6a5b0..cde19faf26 100644
--- a/pcbnew/tools/zone_create_helper.cpp
+++ b/pcbnew/tools/zone_create_helper.cpp
@@ -21,6 +21,8 @@
  * 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA
  */
 
+#include <core/spinlock.h>
+#include <connectivity/connectivity_data.h>
 #include <tools/zone_create_helper.h>
 #include <tool/tool_manager.h>
 #include <zone.h>
@@ -168,6 +170,8 @@ void ZONE_CREATE_HELPER::performZoneCutout( ZONE& aZone, ZONE& aCutout )
 
     ZONE_FILLER filler( board, &commit );
 
+    std::lock_guard<KISPINLOCK> lock( board->GetConnectivity()->GetLock() );
+
     if( !filler.Fill( newZones ) )
     {
         commit.Revert();
@@ -207,9 +211,12 @@ void ZONE_CREATE_HELPER::commitZone( std::unique_ptr<ZONE> aZone )
             aZone->HatchBorder();
             bCommit.Add( aZone.get() );
 
+            BOARD* board = m_tool.getModel<BOARD>();
+            std::lock_guard<KISPINLOCK> lock( board->GetConnectivity()->GetLock() );
+
             if( !m_params.m_keepout )
             {
-                ZONE_FILLER filler( m_tool.getModel<BOARD>(), &bCommit );
+                ZONE_FILLER filler( board, &bCommit );
                 std::vector<ZONE*> toFill = { aZone.get() };
 
                 if( !filler.Fill( toFill ) )
diff --git a/pcbnew/tools/zone_filler_tool.cpp b/pcbnew/tools/zone_filler_tool.cpp
index 99da06d7f0..2a8128e35a 100644
--- a/pcbnew/tools/zone_filler_tool.cpp
+++ b/pcbnew/tools/zone_filler_tool.cpp
@@ -128,6 +128,8 @@ void ZONE_FILLER_TOOL::FillAllZones( wxWindow* aCaller, PROGRESS_REPORTER* aRepo
     else
         filler.InstallNewProgressReporter( aCaller, _( "Fill All Zones" ), 3 );
 
+    std::lock_guard<KISPINLOCK> lock( board()->GetConnectivity()->GetLock() );
+
     if( filler.Fill( toFill ) )
     {
         commit.Push( _( "Fill Zone(s)" ), false );
@@ -171,6 +173,8 @@ int ZONE_FILLER_TOOL::ZoneFill( const TOOL_EVENT& aEvent )
     ZONE_FILLER filler( board(), &commit );
     filler.InstallNewProgressReporter( frame(), _( "Fill Zone" ), 4 );
 
+    std::lock_guard<KISPINLOCK> lock( board()->GetConnectivity()->GetLock() );
+
     if( filler.Fill( toFill ) )
         commit.Push( _( "Fill Zone(s)" ), false );
     else
diff --git a/pcbnew/zone_filler.cpp b/pcbnew/zone_filler.cpp
index 8e123ae159..2063e7ab4e 100644
--- a/pcbnew/zone_filler.cpp
+++ b/pcbnew/zone_filler.cpp
@@ -88,10 +88,6 @@ bool ZONE_FILLER::Fill( std::vector<ZONE*>& aZones, bool aCheck, wxWindow* aPare
     std::vector<CN_ZONE_ISOLATED_ISLAND_LIST> islandsList;
 
     std::shared_ptr<CONNECTIVITY_DATA> connectivity = m_board->GetConnectivity();
-    std::unique_lock<std::mutex> lock( connectivity->GetLock(), std::try_to_lock );
-
-    if( !lock )
-        return false;
 
     // Rebuild just in case. This really needs to be reliable.
     connectivity->Clear();
diff --git a/pcbnew/zone_filler.h b/pcbnew/zone_filler.h
index 80dd840d5c..8fa0641418 100644
--- a/pcbnew/zone_filler.h
+++ b/pcbnew/zone_filler.h
@@ -44,6 +44,13 @@ public:
 
     void SetProgressReporter( PROGRESS_REPORTER* aReporter );
     void InstallNewProgressReporter( wxWindow* aParent, const wxString& aTitle, int aNumPhases );
+
+    /**
+     * Fills the given list of zones.  Invalidates connectivity - it is up to the caller to obtain
+     * a lock on the connectivity data before calling Fill to prevent access to stale data by other
+     * coroutines (for example, ratsnest redraw).  This will generally be required if a UI-based
+     * progress reporter has been installed.
+     */
     bool Fill( std::vector<ZONE*>& aZones, bool aCheck = false, wxWindow* aParent = nullptr );
 
     bool IsDebug() const { return m_debugZoneFiller; }