IOFiber.scala.diff

diff --git a/core/shared/src/main/scala/cats/effect/IOFiber.scala b/core/shared/src/main/scala/cats/effect/IOFiber.scala
index ea00de579..0af2fde00 100644
--- a/core/shared/src/main/scala/cats/effect/IOFiber.scala
+++ b/core/shared/src/main/scala/cats/effect/IOFiber.scala
@@ -26,7 +26,7 @@ import scala.concurrent.duration._
 import scala.util.control.NonFatal
 
 import java.util.concurrent.RejectedExecutionException
-import java.util.concurrent.atomic.AtomicBoolean
+import java.util.concurrent.atomic.{AtomicBoolean, AtomicInteger}
 import scala.util.control.NoStackTrace
 
 /*
@@ -112,7 +112,8 @@ private final class IOFiber[A](
   private[this] val TypeBlocking = Sync.Type.Blocking
 
   /* mutable state for resuming the fiber in different states */
-  private[this] var resumeTag: Byte = ExecR
+  private[this] val resumeTag = new AtomicInteger(ExecR)
+
   private[this] var resumeIO: IO[Any] = startIO
 
   /* prefetch for Right(()) */
@@ -128,10 +129,10 @@ private final class IOFiber[A](
   private[this] val autoYieldThreshold = runtime.config.autoYieldThreshold
 
   override def run(): Unit = {
-    // insert a read barrier after every async boundary
+    // insert a read barrier after every async boundary (TODO do we still need this?)
     readBarrier()
     try {
-      (resumeTag: @switch) match {
+      (resumeTag.getAndSet(DoneR): @switch) match {
         case 0 => execR()
         case 1 => asyncContinueSuccessfulR()
         case 2 => asyncContinueFailedR()
@@ -238,7 +239,7 @@ private final class IOFiber[A](
       asyncCancel(null)
     } else if (iteration >= autoYieldThreshold) {
       resumeIO = cur0
-      resumeTag = AutoCedeR
+      resumeTag.set(AutoCedeR)
       rescheduleFiber(currentCtx)(this)
     } else {
       // This is a modulo operation in disguise. `iteration` is reset every time
@@ -568,10 +569,10 @@ private final class IOFiber[A](
                     val ec = currentCtx
                     e match {
                       case Left(t) =>
-                        resumeTag = AsyncContinueFailedR
+                        resumeTag.set(AsyncContinueFailedR)
                         objectState.push(t)
                       case Right(a) =>
-                        resumeTag = AsyncContinueSuccessfulR
+                        resumeTag.set(AsyncContinueSuccessfulR)
                         objectState.push(a.asInstanceOf[AnyRef])
                     }
                     execute(ec)(this)
@@ -749,7 +750,7 @@ private final class IOFiber[A](
 
         /* Cede */
         case 16 =>
-          resumeTag = CedeR
+          resumeTag.set(CedeR)
           rescheduleFiber(currentCtx)(this)
 
         case 17 =>
@@ -795,7 +796,7 @@ private final class IOFiber[A](
             ctxs.push(ec)
             conts.push(EvalOnK)
 
-            resumeTag = EvalOnR
+            resumeTag.set(EvalOnR)
             resumeIO = cur.ioa
             execute(ec)(this)
           }
@@ -805,7 +806,7 @@ private final class IOFiber[A](
           /* we know we're on the JVM here */
 
           if (cur.hint eq TypeBlocking) {
-            resumeTag = BlockingR
+            resumeTag.set(BlockingR)
             resumeIO = cur
             runtime.blocking.execute(this)
           } else {
@@ -838,7 +839,7 @@ private final class IOFiber[A](
      */
     masks = initMask
 
-    resumeTag = DoneR
+    resumeTag.set(DoneR)
     resumeIO = null
     /*
      * Write barrier to publish masks. The thread which owns the runloop is
@@ -1093,10 +1094,10 @@ private final class IOFiber[A](
       }
 
     if (error == null) {
-      resumeTag = AfterBlockingSuccessfulR
+      resumeTag.set(AfterBlockingSuccessfulR)
       objectState.push(r.asInstanceOf[AnyRef])
     } else {
-      resumeTag = AfterBlockingFailedR
+      resumeTag.set(AfterBlockingFailedR)
       objectState.push(error)
     }
     currentCtx.execute(this)
@@ -1198,7 +1199,7 @@ private final class IOFiber[A](
     val ec = popContext()
 
     if (!shouldFinalize()) {
-      resumeTag = AfterBlockingSuccessfulR
+      resumeTag.set(AfterBlockingSuccessfulR)
       objectState.push(result.asInstanceOf[AnyRef])
       execute(ec)(this)
     } else {
@@ -1212,7 +1213,7 @@ private final class IOFiber[A](
     val ec = popContext()
 
     if (!shouldFinalize()) {
-      resumeTag = AfterBlockingFailedR
+      resumeTag.set(AfterBlockingFailedR)
       objectState.push(t)
       execute(ec)(this)
     } else {