Switch Worker into a Blocking mode if it tries to run runBlocking wit…

…h a CPU permit

And reacquire CPU permit after runBlocking finishes. This should resolve Dispatchers.Default starvation in cases where runBlocking is used to run suspend functions from non-suspend execution context.

Kotlin#3983 / IJPL-721

kotlinx-coroutines-core/jvm/src/Builders.kt

@@ -4,6 +4,8 @@
 
 package kotlinx.coroutines
 
+import kotlinx.coroutines.scheduling.*
+import kotlinx.coroutines.scheduling.CoroutineScheduler
 import java.util.concurrent.locks.*
 import kotlin.contracts.*
 import kotlin.coroutines.*
@@ -86,6 +88,7 @@ private class BlockingCoroutine<T>(
     @Suppress("UNCHECKED_CAST")
     fun joinBlocking(): T {
         registerTimeLoopThread()
+        var cpuPermitReleased: Boolean? = null
         try {
             eventLoop?.incrementUseCount()
             try {
@@ -95,13 +98,20 @@ private class BlockingCoroutine<T>(
                     val parkNanos = eventLoop?.processNextEvent() ?: Long.MAX_VALUE
                     // note: process next even may loose unpark flag, so check if completed before parking
                     if (isCompleted) break
+                    if (parkNanos > 0 && cpuPermitReleased == null) {
+                        val worker = Thread.currentThread() as? CoroutineScheduler.Worker
+                        cpuPermitReleased = worker?.releaseCpu() ?: false
+                    }
                     parkNanos(this, parkNanos)
                 }
             } finally { // paranoia
                 eventLoop?.decrementUseCount()
             }
         } finally { // paranoia
             unregisterTimeLoopThread()
+            if (cpuPermitReleased == true) {
+                (Thread.currentThread() as CoroutineScheduler.Worker).reacquireCpu()
+            }
         }
         // now return result
         val state = this.state.unboxState()

kotlinx-coroutines-core/jvm/src/scheduling/CoroutineScheduler.kt

@@ -690,6 +690,55 @@ internal class CoroutineScheduler(
             return hadCpu
         }
 
+        /** only for [runBlocking] */
+        fun releaseCpu(): Boolean {
+            assert { state == WorkerState.CPU_ACQUIRED || state == WorkerState.BLOCKING }
+            return tryReleaseCpu(WorkerState.BLOCKING).also {
+                if (it) {
+                    incrementBlockingTasks()
+                }
+            }
+        }
+
+        /** only for [runBlocking] */
+        fun reacquireCpu() {
+            assert { state == WorkerState.BLOCKING }
+            decrementBlockingTasks()
+            if (tryAcquireCpuPermit()) return
+            class CpuPermitTransfer {
+                private val status = atomic(false)
+                fun check(): Boolean = status.value
+                fun complete(): Boolean = status.compareAndSet(false, true)
+            }
+            val permitTransfer = CpuPermitTransfer()
+            val blockedWorker = this@Worker
+            scheduler.dispatch(Runnable {
+                // this code runs in a different worker thread that holds a CPU token
+                val cpuHolder = currentThread() as Worker
+                assert { cpuHolder.state == WorkerState.CPU_ACQUIRED }
+                if (permitTransfer.complete()) {
+                    cpuHolder.state = WorkerState.BLOCKING
+                    LockSupport.unpark(blockedWorker)
+                }
+            }, taskContext = NonBlockingContext)
+            state = WorkerState.PARKING
+            while (true) {
+                if (permitTransfer.check()) {
+                    state = WorkerState.CPU_ACQUIRED
+                    break
+                }
+                if (tryAcquireCpuPermit()) {
+                    if (!permitTransfer.complete()) {
+                        // race: transfer was completed by another thread
+                        releaseCpuPermit()
+                    }
+                    assert { state == WorkerState.CPU_ACQUIRED }
+                    break
+                }
+                LockSupport.parkNanos(RUN_BLOCKING_CPU_REACQUIRE_PARK_NS)
+            }
+        }
+
         override fun run() = runWorker()
 
         @JvmField

kotlinx-coroutines-core/jvm/src/scheduling/Tasks.kt

@@ -19,6 +19,11 @@ internal val WORK_STEALING_TIME_RESOLUTION_NS = systemProp(
     "kotlinx.coroutines.scheduler.resolution.ns", 100000L
 )
 
+@JvmField
+internal val RUN_BLOCKING_CPU_REACQUIRE_PARK_NS = systemProp(
+    "kotlinx.coroutines.scheduler.runBlocking.cpu.reacquire.ns", 250L * 1000 * 1000
+)
+
 /**
  * The maximum number of threads allocated for CPU-bound tasks at the default set of dispatchers.
  *

kotlinx-coroutines-core/jvm/test/scheduling/BlockingCoroutineDispatcherTest.kt

@@ -1,6 +1,5 @@
 package kotlinx.coroutines.scheduling
 
-import kotlinx.coroutines.testing.*
 import kotlinx.coroutines.*
 import org.junit.*
 import org.junit.rules.*
@@ -171,4 +170,20 @@ class BlockingCoroutineDispatcherTest : SchedulerTestBase() {
     fun testZeroParallelism() {
         blockingDispatcher(0)
     }
+
+    @Test
+    fun testNoCpuStarvationWithDeepRunBlocking() {
+        val maxDepth = CORES_COUNT * 3 + 3
+        fun body(depth: Int) {
+            if (depth == maxDepth) return
+            runBlocking(dispatcher) {
+                launch(dispatcher) {
+                    body(depth + 1)
+                }
+            }
+        }
+
+        body(1)
+        checkPoolThreadsCreated(maxDepth..maxDepth + 1)
+    }
 }