~ Get rid of loop in addAcquireToQueue

Author: elizarov

kotlinx-coroutines-core/common/src/sync/Semaphore.kt

@@ -189,23 +189,25 @@ private class SemaphoreImpl(private val permits: Int, acquiredPermits: Int) : Se
         val segment = this.tail.findSegmentAndMoveForward(id = enqIdx / SEGMENT_SIZE, startFrom = curTail,
             createNewSegment = ::createSegment).segment // cannot be closed
         val i = (enqIdx % SEGMENT_SIZE).toInt()
-        while (true) { // cas loop on cell state
-            val cellState = segment.get(i)
-            when {
-                cellState === null -> // the cell if empty, try to install continuation
-                    if (segment.cas(i, null, cont)) { // fast path -- installed continuation successfully
-                        cont.invokeOnCancellation(CancelSemaphoreAcquisitionHandler(segment, i).asHandler)
-                        return true
-                    }
-                cellState === PERMIT -> // the cell already has permit from tryResumeNextFromQueue, try to grab it
-                    if (segment.cas(i, PERMIT, TAKEN)) { // took permit thus eliminating acquire/release pair
-                        cont.resume(Unit)
-                        return true
-                    }
-                cellState === BROKEN -> return false // broken cell, need to retry on a different cell
-                else -> error("Invalid state $cellState") // this cannot happen
+        var cellState = segment.get(i)
+        // Fast path -- if the cell is empty, try to install continuation
+        if (cellState === null) {
+            if (segment.cas(i, null, cont)) { // installed continuation successfully
+                cont.invokeOnCancellation(CancelSemaphoreAcquisitionHandler(segment, i).asHandler)
+                return true
             }
+            cellState = segment.get(i) // Reread on CAS failure -- it was updated to either PERMIT or BROKEN
         }
+        // the cell already has permit from tryResumeNextFromQueue, try to grab it
+        if (cellState === PERMIT) {
+            if (segment.cas(i, PERMIT, TAKEN)) { // took permit thus eliminating acquire/release pair
+                cont.resume(Unit)
+                return true
+            }
+            cellState = segment.get(i) // Reread on CAS failure -- it could be update to BROKEN
+        }
+        check(cellState === BROKEN) { "Invalid state $cellState" }
+        return false // broken cell, need to retry on a different cell
     }
 
     @Suppress("UNCHECKED_CAST")