isLeftmostOrProcessed in Semaphore

Author: de-shyt

kotlinx-coroutines-core/common/src/channels/BufferedChannel.kt

@@ -90,6 +90,10 @@ internal open class BufferedChannel<E>(
     private val receiveSegment: AtomicRef<ChannelSegment<E>>
     private val bufferEndSegment: AtomicRef<ChannelSegment<E>>
 
+    /**
+      These values are used in [ChannelSegment.isLeftmostOrProcessed].
+      They help to detect when the `prev` reference of the segment should be cleaned.
+     */
     internal val sendSegmentId: Long get() = sendSegment.value.id
     internal val receiveSegmentId: Long get() = receiveSegment.value.id
 
@@ -146,9 +150,9 @@ internal open class BufferedChannel<E>(
         the segment and the index in it. */
         segment: ChannelSegment<E>,
         index: Int,
-        /** The element to be inserted. */
+        /* The element to be inserted. */
         element: E,
-        /** The global index of the cell. */
+        /* The global index of the cell. */
         s: Long
     ) = suspendCancellableCoroutineReusable sc@{ cont ->
         sendImplOnNoWaiter( // <-- this is an inline function

kotlinx-coroutines-core/common/src/internal/ConcurrentLinkedList.kt

@@ -71,7 +71,7 @@ internal inline fun <S : Segment<S>> AtomicRef<S>.moveForward(to: S): Boolean =
  *
  * The method is called when [moveForward] successfully updates the value stored in the `AtomicRef` reference.
  */
-internal inline fun <S : Segment<S>> cleanLeftmostPrev(from: S, to: S) {
+private inline fun <S : Segment<S>> cleanLeftmostPrev(from: S, to: S) {
     var cur = to
     // Find the leftmost segment on the sublist between `from` and `to` segments.
     while (!cur.isLeftmostOrProcessed && cur.id > from.id) {

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

@@ -128,10 +128,16 @@ internal open class SemaphoreAndMutexImpl(private val permits: Int, acquiredPerm
     private val tail: AtomicRef<SemaphoreSegment>
     private val enqIdx = atomic(0L)
 
+    /**
+      This value is used in [SemaphoreSegment.isLeftmostOrProcessed].
+      It helps to detect when the `prev` reference of the segment should be cleaned.
+    */
+    internal val headId: Long get() = head.value.id
+
     init {
         require(permits > 0) { "Semaphore should have at least 1 permit, but had $permits" }
         require(acquiredPermits in 0..permits) { "The number of acquired permits should be in 0..$permits" }
-        val s = SemaphoreSegment(0, null)
+        val s = SemaphoreSegment(0, null, this)
         head = atomic(s)
         tail = atomic(s)
     }
@@ -317,7 +323,6 @@ internal open class SemaphoreAndMutexImpl(private val permits: Int, acquiredPerm
         val createNewSegment = ::createSegment
         val segment = this.head.findSegmentAndMoveForward(id, startFrom = curHead,
             createNewSegment = createNewSegment).segment // cannot be closed
-        segment.cleanPrev()
         if (segment.id > id) return false
         val i = (deqIdx % SEGMENT_SIZE).toInt()
         val cellState = segment.getAndSet(i, PERMIT) // set PERMIT and retrieve the prev cell state
@@ -356,12 +361,19 @@ private class SemaphoreImpl(
     permits: Int, acquiredPermits: Int
 ): SemaphoreAndMutexImpl(permits, acquiredPermits), Semaphore
 
-private fun createSegment(id: Long, prev: SemaphoreSegment?) = SemaphoreSegment(id, prev)
+private fun createSegment(id: Long, prev: SemaphoreSegment) = SemaphoreSegment(
+    id = id,
+    prev = prev,
+    semaphore = prev.semaphore
+)
 
-private class SemaphoreSegment(id: Long, prev: SemaphoreSegment?) : Segment<SemaphoreSegment>(id, prev) {
+private class SemaphoreSegment(
+    id: Long, prev: SemaphoreSegment?,
+    val semaphore: SemaphoreAndMutexImpl
+) : Segment<SemaphoreSegment>(id, prev) {
     val acquirers = atomicArrayOfNulls<Any?>(SEGMENT_SIZE)
     override val numberOfSlots: Int get() = SEGMENT_SIZE
-    override val isLeftmostOrProcessed: Boolean get() = false // Does not impact semaphore implementation
+    override val isLeftmostOrProcessed: Boolean get() = id <= semaphore.headId
 
     @Suppress("NOTHING_TO_INLINE")
     inline fun get(index: Int): Any? = acquirers[index].value