add preliminary semaphore implementation

Signed-off-by: leongross <leon.gross@9elements.com>

src/runtime/sync.go

@@ -1,17 +1,72 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
 package runtime
 
+import (
+	"sync/atomic"
+	"unsafe"
+)
+
 // This file contains stub implementations for internal/poll.
+// The official golang implementation states:
+//
+// "That is, don't think of these as semaphores.
+// Think of them as a way to implement sleep and wakeup
+// such that every sleep is paired with a single wakeup,
+// even if, due to races, the wakeup happens before the sleep."
+//
+// This is an experimental and probably incomplete implementation of the
+// semaphore system, tailed to the network use case. That means, that it does not
+// implement the modularity that the semacquire/semacquire1 implementation model
+// offers, which in fact is emitted here entirely.
+// This means we assume the following constant settings from the golang standard
+// library: lifo=false,profile=semaBlock,skipframe=0,reason=waitReasonSemaquire
+
+type semaRoot struct {
+	nwait atomic.Uint32
+}
+
+var semtable semTable
+
+// Prime to not correlate with any user patterns.
+const semTabSize = 251
+
+type semTable [semTabSize]struct {
+	root semaRoot
+	pad  [64 - unsafe.Sizeof(semaRoot{})]byte // only 64 x86_64, make this variable
+}
+
+func (t *semTable) rootFor(addr *uint32) *semaRoot {
+	return &t[(uintptr(unsafe.Pointer(addr))>>3)%semTabSize].root
+}
 
 //go:linkname semacquire internal/poll.runtime_Semacquire
 func semacquire(sema *uint32) {
-	// TODO the "net" pkg calls this, so panic() isn't an option.  Right
-	// now, just ignore the call.
-	// panic("todo: semacquire")
+	if cansemacquire(sema) {
+		return
+	}
+}
+
+// Copied from src/runtime/sema.go
+func cansemacquire(addr *uint32) bool {
+	for {
+		v := atomic.LoadUint32(addr)
+		if v == 0 {
+			return false
+		}
+		if atomic.CompareAndSwapUint32(addr, v, v-1) {
+			return true
+		}
+	}
 }
 
 //go:linkname semrelease internal/poll.runtime_Semrelease
 func semrelease(sema *uint32) {
-	// TODO the "net" pkg calls this, so panic() isn't an option.  Right
-	// now, just ignore the call.
-	// panic("todo: semrelease")
+	root := semtable.rootFor(sema)
+	atomic.AddUint32(sema, 1)
+	if root.nwait.Load() == 0 {
+		return
+	}
 }