Make tryAcquire linearizable

Author: ndkoval

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

@@ -113,9 +113,24 @@ private class SemaphoreImpl(
     private val deqIdx = atomic(0L)
 
     override fun tryAcquire(): Boolean {
-        _availablePermits.loop { p ->
-            if (p <= 0) return false
-            if (_availablePermits.compareAndSet(p, p - 1)) return true
+        while (true) {
+            val p = _availablePermits.value
+            if (p > 0) {
+                // try to get a permit by updating the counter
+                if (_availablePermits.compareAndSet(p,  p - 1)) return true
+            } else {
+                // check whether there is a permit in the queue
+                val enqIdx = this.enqIdx.value
+                val deqIdx = this.deqIdx.value
+                if (enqIdx >= deqIdx) return false
+                // try to get a permit from the queue
+                if (this.enqIdx.compareAndSet(enqIdx, enqIdx + 1)) {
+                    // update the counter of available permits and check
+                    // whether it was legal to get a permit from the queue
+                    if (_availablePermits.getAndDecrement() <= 0) return true
+                    else release()
+                }
+            }
         }
     }