diff --git a/classes/exception/ExecutionOutsideLockException.php b/classes/exception/ExecutionOutsideLockException.php
new file mode 100644
index 00000000..6075b955
--- /dev/null
+++ b/classes/exception/ExecutionOutsideLockException.php
@@ -0,0 +1,16 @@
+<?php
+
+namespace malkusch\lock\exception;
+
+/**
+ * This exception should be thrown when for example the lock is released or
+ * times out before the synchronized code finished execution.
+ *
+ * @see \malkusch\lock\mutex\SpinlockMutex::unlock()
+ *
+ * @author Petr Levtonov <petr@levtonov.com>
+ * @license WTFPL
+ */
+class ExecutionOutsideLockException extends LockReleaseException
+{
+}
diff --git a/classes/mutex/SpinlockMutex.php b/classes/mutex/SpinlockMutex.php
index fd713b2f..f921687d 100644
--- a/classes/mutex/SpinlockMutex.php
+++ b/classes/mutex/SpinlockMutex.php
@@ -2,9 +2,10 @@
 
 namespace malkusch\lock\mutex;
 
-use malkusch\lock\util\Loop;
-use malkusch\lock\exception\LockReleaseException;
+use malkusch\lock\exception\ExecutionOutsideLockException;
 use malkusch\lock\exception\LockAcquireException;
+use malkusch\lock\exception\LockReleaseException;
+use malkusch\lock\util\Loop;
 
 /**
  * Spinlock implementation.
@@ -79,11 +80,13 @@ protected function unlock()
         $elapsed = microtime(true) - $this->acquired;
         if ($elapsed >= $this->timeout) {
             $message = sprintf(
-                "The code executed for %d seconds. But the timeout is %d seconds.",
+                "The code executed for %.2F seconds. But the timeout is %d " .
+                "seconds. The last %.2F seconds were executed outside the lock.",
                 $elapsed,
-                $this->timeout
+                $this->timeout,
+                $elapsed - $this->timeout
             );
-            throw new LockReleaseException($message);
+            throw new ExecutionOutsideLockException($message);
         }
 
         /*
diff --git a/tests/mutex/SpinlockMutexTest.php b/tests/mutex/SpinlockMutexTest.php
index f1038855..3e8607cf 100644
--- a/tests/mutex/SpinlockMutexTest.php
+++ b/tests/mutex/SpinlockMutexTest.php
@@ -2,9 +2,10 @@
 
 namespace malkusch\lock\mutex;
 
+use malkusch\lock\exception\ExecutionOutsideLockException;
 use malkusch\lock\exception\LockAcquireException;
-use phpmock\phpunit\PHPMock;
 use phpmock\environment\SleepEnvironmentBuilder;
+use phpmock\phpunit\PHPMock;
 
 /**
  * Tests for SpinlockMutex.
@@ -68,7 +69,6 @@ public function testAcquireTimesOut()
      * Tests executing code which exceeds the timeout fails.
      *
      * @test
-     * @expectedException malkusch\lock\exception\LockReleaseException
      */
     public function testExecuteTooLong()
     {
@@ -76,6 +76,12 @@ public function testExecuteTooLong()
         $mutex->expects($this->any())->method("acquire")->willReturn(true);
         $mutex->expects($this->any())->method("release")->willReturn(true);
 
+        $this->expectException(ExecutionOutsideLockException::class);
+        $this->expectExceptionMessageRegExp(
+            '/The code executed for \d+\.\d+ seconds. But the timeout is 1 ' .
+            'seconds. The last \d+\.\d+ seconds were executed outside the lock./'
+        );
+
         $mutex->synchronized(function () {
             sleep(1);
         });