Added comment that LockFreeMPSCQueue is not linearizable,

removed the corresponding linearizability test.

Author: elizarov

core/kotlinx-coroutines-core/src/main/kotlin/kotlinx/coroutines/experimental/internal/LockFreeMPSCQueue.kt

@@ -22,6 +22,16 @@ import java.util.concurrent.atomic.*
 private typealias Core<E> = LockFreeMPSCQueueCore<E>
 
 /**
+ * Lock-free Multiply-Producer Single-Consumer Queue.
+ * *Note: This queue is NOT linearizable. It provides only quiescent consistency for its operations.*
+ *
+ * In particular, the following execution is permitted for this queue, but is not permitted for a linearizable queue:
+ *
+ * ```
+ * Thread 1: addLast(1) = true, removeFirstOrNull() = null
+ * Thread 2: addLast(2) = 2 // this operation is concurrent with both operations in the first thread
+ * ```
+ *
  * @suppress **This is unstable API and it is subject to change.**
  */
 internal class LockFreeMPSCQueue<E : Any> {
@@ -58,6 +68,10 @@ internal class LockFreeMPSCQueue<E : Any> {
 }
 
 /**
+ * Lock-free Multiply-Producer Single-Consumer Queue core.
+ * *Note: This queue is NOT linearizable. It provides only quiescent consistency for its operations.*
+ *
+ * @see LockFreeMPSCQueue
  * @suppress **This is unstable API and it is subject to change.**
  */
 internal class LockFreeMPSCQueueCore<E : Any>(private val capacity: Int) {

core/kotlinx-coroutines-core/src/test/kotlin/kotlinx/coroutines/experimental/internal/LockFreeMPSCQueueLinearizabilityTest.kt

@@ -39,14 +39,18 @@ class LockFreeMPSCQueueLinearizabilityTest : TestBase() {
     @Operation
     fun addLast(@Param(name = "value") value: Int) = q.addLast(value)
 
-    @Operation(group = "consumer")
-    fun removeFirstOrNull() = q.removeFirstOrNull()
+    /**
+     * Note, that removeFirstOrNull is not linearizable w.r.t. to addLast, so here
+     * we test only linearizability of close.
+     */
+//    @Operation(group = "consumer")
+//    fun removeFirstOrNull() = q.removeFirstOrNull()
 
     @Test
     fun testLinearizability() {
         val options = StressOptions()
-            .iterations(100)
-            .invocationsPerIteration(1000 * stressTestMultiplier)
+            .iterations(100 * stressTestMultiplierSqrt)
+            .invocationsPerIteration(1000 * stressTestMultiplierSqrt)
             .addThread(1, 3)
             .addThread(1, 3)
             .addThread(1, 3)