diff --git a/libs/image/tiles3/KisTiledExtentManager.cpp b/libs/image/tiles3/KisTiledExtentManager.cpp
index c326c0208c..b1d1427ff5 100644
--- a/libs/image/tiles3/KisTiledExtentManager.cpp
+++ b/libs/image/tiles3/KisTiledExtentManager.cpp
@@ -1,339 +1,347 @@
 /*
  *  Copyright (c) 2017 Dmitry Kazakov <dimula73@gmail.com>
  *  Copyright (c) 2018 Andrey Kamakin <a.kamakin@icloud.com>
  *
  *  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 2 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, write to the Free Software
  *  Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
  */
 
 #include "KisTiledExtentManager.h"
 
 #include <QMutex>
 #include <QVector>
 #include "kis_tile_data_interface.h"
 #include "kis_assert.h"
 #include "kis_global.h"
 #include "kis_debug.h"
 
 KisTiledExtentManager::Data::Data()
     : m_min(qint32_MAX), m_max(qint32_MIN), m_count(0)
 {
     QWriteLocker lock(&m_migrationLock);
     m_capacity = InitialBufferSize;
     m_offset = 1;
     m_buffer = new QAtomicInt[m_capacity];
 }
 
 KisTiledExtentManager::Data::~Data()
 {
     QWriteLocker lock(&m_migrationLock);
     delete[] m_buffer;
 }
 
 bool KisTiledExtentManager::Data::add(qint32 index)
 {
     QReadLocker lock(&m_migrationLock);
     qint32 currentIndex = m_offset + index;
 
     if (currentIndex < 0 || currentIndex >= m_capacity) {
         lock.unlock();
         migrate(index);
         lock.relock();
         currentIndex = m_offset + index;
     }
 
     KIS_ASSERT_RECOVER_NOOP(m_buffer[currentIndex].loadAcquire() >= 0);
     bool needsUpdateExtent = false;
     QReadLocker rl(&m_extentLock);
 
     if (!m_buffer[currentIndex].loadAcquire()) {
         rl.unlock();
         QWriteLocker wl(&m_extentLock);
 
         if (!m_buffer[currentIndex].load()) {
             m_buffer[currentIndex].store(1);
 
             if (m_min > index) m_min = index;
             if (m_max < index) m_max = index;
 
             ++m_count;
             needsUpdateExtent = true;
         } else {
             m_buffer[currentIndex].ref();
         }
     } else {
         m_buffer[currentIndex].ref();
     }
 
     return needsUpdateExtent;
 }
 
 bool KisTiledExtentManager::Data::remove(qint32 index)
 {
     QReadLocker lock(&m_migrationLock);
     qint32 currentIndex = m_offset + index;
 
+    /**
+     * That is not the droid you're looking for. If you see this assert
+     * in the backtrace, most probably, the bug is not here. The crash
+     * happens because two threads are trying to do device->clear(rc)
+     * concurrently for the overlapping rects. That is, they are trying
+     * to remove the same tile. Look higher!
+     */
     KIS_ASSERT_RECOVER_NOOP(m_buffer[currentIndex].loadAcquire() > 0);
+
     bool needsUpdateExtent = false;
     QReadLocker rl(&m_extentLock);
 
     if (m_buffer[currentIndex].loadAcquire() == 1) {
         rl.unlock();
         QWriteLocker wl(&m_extentLock);
 
         if (m_buffer[currentIndex].load() == 1) {
             m_buffer[currentIndex].store(0);
 
             if (m_min == index) updateMin();
             if (m_max == index) updateMax();
 
             --m_count;
             needsUpdateExtent = true;
         } else {
             m_buffer[currentIndex].deref();
         }
     } else {
         const bool nonZero = m_buffer[currentIndex].deref();
 
         /**
          * BUG: since we are not holding the write lock, it might happen
          * that two threads failed the original 'if' and entered this
          * branch. It means that the device will report unexistent
          * extent... See KisTiledDataManagerTest::stressTestExtentsColumn()
          * for reproduction.
          *
          * The same bug is present in KisTiledExtentManager::Data::add()
          */
         KIS_SAFE_ASSERT_RECOVER_NOOP(nonZero);
     }
 
     return needsUpdateExtent;
 }
 
 void KisTiledExtentManager::Data::replace(const QVector<qint32> &indexes)
 {
     QWriteLocker lock(&m_migrationLock);
     QWriteLocker l(&m_extentLock);
 
     for (qint32 i = 0; i < m_capacity; ++i) {
         m_buffer[i].store(0);
     }
 
     m_min = qint32_MAX;
     m_max = qint32_MIN;
     m_count = 0;
 
     Q_FOREACH (const qint32 index, indexes) {
         unsafeAdd(index);
     }
 }
 
 void KisTiledExtentManager::Data::clear()
 {
     QWriteLocker lock(&m_migrationLock);
     QWriteLocker l(&m_extentLock);
 
     for (qint32 i = 0; i < m_capacity; ++i) {
         m_buffer[i].store(0);
     }
 
     m_min = qint32_MAX;
     m_max = qint32_MIN;
     m_count = 0;
 }
 
 bool KisTiledExtentManager::Data::isEmpty()
 {
     return m_count == 0;
 }
 
 qint32 KisTiledExtentManager::Data::min()
 {
     return m_min;
 }
 
 qint32 KisTiledExtentManager::Data::max()
 {
     return m_max;
 }
 
 void KisTiledExtentManager::Data::unsafeAdd(qint32 index)
 {
     qint32 currentIndex = m_offset + index;
 
     if (currentIndex < 0 || currentIndex >= m_capacity) {
         unsafeMigrate(index);
         currentIndex = m_offset + index;
     }
 
     if (!m_buffer[currentIndex].fetchAndAddRelaxed(1)) {
         if (m_min > index) m_min = index;
         if (m_max < index) m_max = index;
         ++m_count;
     }
 }
 
 void KisTiledExtentManager::Data::unsafeMigrate(qint32 index)
 {
     qint32 oldCapacity = m_capacity;
     qint32 oldOffset = m_offset;
     qint32 currentIndex = m_offset + index;
 
     while (currentIndex < 0 || currentIndex >= m_capacity) {
         m_capacity <<= 1;
 
         if (currentIndex < 0) {
             m_offset <<= 1;
             currentIndex = m_offset + index;
         }
     }
 
     if (m_capacity != oldCapacity) {
         QAtomicInt *newBuffer = new QAtomicInt[m_capacity];
         qint32 start = m_offset - oldOffset;
 
         for (qint32 i = 0; i < oldCapacity; ++i) {
             newBuffer[start + i].store(m_buffer[i].load());
         }
 
         delete[] m_buffer;
         m_buffer = newBuffer;
     }
 }
 
 void KisTiledExtentManager::Data::migrate(qint32 index)
 {
     QWriteLocker lock(&m_migrationLock);
     unsafeMigrate(index);
 }
 
 void KisTiledExtentManager::Data::updateMin()
 {
     qint32 start = m_min + m_offset;
 
     for (qint32 i = start; i < m_capacity; ++i) {
         qint32 current = m_buffer[i].load();
 
         if (current > 0) {
             m_min = i - m_offset;
             break;
         }
     }
 }
 
 void KisTiledExtentManager::Data::updateMax()
 {
     qint32 start = m_max + m_offset;
 
     for (qint32 i = start; i >= 0; --i) {
         qint32 current = m_buffer[i].load();
 
         if (current > 0) {
             m_max = i - m_offset;
             break;
         }
     }
 }
 
 KisTiledExtentManager::KisTiledExtentManager()
 {
     QWriteLocker l(&m_extentLock);
     m_currentExtent = QRect();
 }
 
 void KisTiledExtentManager::notifyTileAdded(qint32 col, qint32 row)
 {
     bool needsUpdateExtent = false;
 
     needsUpdateExtent |= m_colsData.add(col);
     needsUpdateExtent |= m_rowsData.add(row);
 
     if (needsUpdateExtent) {
         updateExtent();
     }
 }
 
 void KisTiledExtentManager::notifyTileRemoved(qint32 col, qint32 row)
 {
     bool needsUpdateExtent = false;
 
     needsUpdateExtent |= m_colsData.remove(col);
     needsUpdateExtent |= m_rowsData.remove(row);
 
     if (needsUpdateExtent) {
         updateExtent();
     }
 }
 
 void KisTiledExtentManager::replaceTileStats(const QVector<QPoint> &indexes)
 {
     QVector<qint32> colsIndexes;
     QVector<qint32> rowsIndexes;
 
     Q_FOREACH (const QPoint &index, indexes) {
         colsIndexes.append(index.x());
         rowsIndexes.append(index.y());
     }
 
     m_colsData.replace(colsIndexes);
     m_rowsData.replace(rowsIndexes);
     updateExtent();
 }
 
 void KisTiledExtentManager::clear()
 {
     m_colsData.clear();
     m_rowsData.clear();
 
     QWriteLocker lock(&m_extentLock);
     m_currentExtent = QRect();
 }
 
 QRect KisTiledExtentManager::extent() const
 {
     QReadLocker lock(&m_extentLock);
     return m_currentExtent;
 }
 
 void KisTiledExtentManager::updateExtent()
 {
     qint32 minX, width, minY, height;
 
     {
         QReadLocker cl(&m_colsData.m_extentLock);
 
         if (m_colsData.isEmpty()) {
             minX = 0;
             width = 0;
         } else {
             minX = m_colsData.min() * KisTileData::WIDTH;
             width = (m_colsData.max() + 1) * KisTileData::WIDTH - minX;
         }
     }
 
     {
         QReadLocker rl(&m_rowsData.m_extentLock);
 
         if (m_rowsData.isEmpty()) {
             minY = 0;
             height = 0;
         } else {
             minY = m_rowsData.min() * KisTileData::HEIGHT;
             height = (m_rowsData.max() + 1) * KisTileData::HEIGHT - minY;
         }
     }
 
     QWriteLocker lock(&m_extentLock);
     m_currentExtent = QRect(minX, minY, width, height);
 }