runtime: convert synchronous semaphores to Go

LGTM=rsc
R=golang-codereviews, khr, rsc
CC=golang-codereviews, rlh
https://golang.org/cl/130340043

src/pkg/runtime/sema.go

+// 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.
+
+// Semaphore implementation exposed to Go.
+// Intended use is provide a sleep and wakeup
+// primitive that can be used in the contended case
+// of other synchronization primitives.
+// Thus it targets the same goal as Linux's futex,
+// but it has much simpler semantics.
+//
+// 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.
+//
+// See Mullender and Cox, ``Semaphores in Plan 9,''
+// http://swtch.com/semaphore.pdf
+
+package runtime
+
+import "unsafe"
+
+// Synchronous semaphore for sync.Cond.
+type syncSema struct {
+	lock lock
+	head *sudog
+	tail *sudog
+}
+
+// Syncsemacquire waits for a pairing syncsemrelease on the same semaphore s.
+func syncsemacquire(s *syncSema) {
+	golock(&s.lock)
+	if s.head != nil && s.head.nrelease > 0 {
+		// Have pending release, consume it.
+		var wake *sudog
+		s.head.nrelease--
+		if s.head.nrelease == 0 {
+			wake = s.head
+			s.head = wake.link
+			if s.head == nil {
+				s.tail = nil
+			}
+		}
+		gounlock(&s.lock)
+		if wake != nil {
+			goready(wake.g)
+		}
+	} else {
+		// Enqueue itself.
+		w := acquireSudog()
+		w.g = getg()
+		w.nrelease = -1
+		w.link = nil
+		w.releasetime = 0
+		t0 := int64(0)
+		if blockprofilerate > 0 {
+			t0 = gocputicks()
+			w.releasetime = -1
+		}
+		if s.tail == nil {
+			s.head = w
+		} else {
+			s.tail.link = w
+		}
+		s.tail = w
+		goparkunlock(&s.lock, "semacquire")
+		if t0 != 0 {
+			goblockevent(int64(w.releasetime)-t0, 2)
+		}
+		releaseSudog(w)
+	}
+}
+
+// Syncsemrelease waits for n pairing syncsemacquire on the same semaphore s.
+func syncsemrelease(s *syncSema, n uint32) {
+	golock(&s.lock)
+	for n > 0 && s.head != nil && s.head.nrelease < 0 {
+		// Have pending acquire, satisfy it.
+		wake := s.head
+		s.head = wake.link
+		if s.head == nil {
+			s.tail = nil
+		}
+		if wake.releasetime != 0 {
+			// TODO: Remove use of unsafe here.
+			releasetimep := (*int64)(unsafe.Pointer(&wake.releasetime))
+			*releasetimep = gocputicks()
+		}
+		goready(wake.g)
+		n--
+	}
+	if n > 0 {
+		// enqueue itself
+		w := acquireSudog()
+		w.g = getg()
+		w.nrelease = int32(n)
+		w.link = nil
+		w.releasetime = 0
+		if s.tail == nil {
+			s.head = w
+		} else {
+			s.tail.link = w
+		}
+		s.tail = w
+		goparkunlock(&s.lock, "semarelease")
+	} else {
+		gounlock(&s.lock)
+	}
+}
+
+func syncsemcheck(sz uintptr) {
+	if sz != unsafe.Sizeof(syncSema{}) {
+		print("runtime: bad syncSema size - sync=", sz, " runtime=", unsafe.Sizeof(syncSema{}), "\n")
+		gothrow("bad syncSema size")
+	}
+}

src/pkg/runtime/sema.goc

@@ -202,93 +202,3 @@ func runtime_Semacquire(addr *uint32) {
 func runtime_Semrelease(addr *uint32) {
 	runtime·semrelease(addr);
 }
-
-typedef struct SyncSema SyncSema;
-struct SyncSema
-{
-	Lock		lock;
-	SemaWaiter*	head;
-	SemaWaiter*	tail;
-};
-
-func runtime_Syncsemcheck(size uintptr) {
-	if(size != sizeof(SyncSema)) {
-		runtime·printf("bad SyncSema size: sync:%D runtime:%D\n", (int64)size, (int64)sizeof(SyncSema));
-		runtime·throw("bad SyncSema size");
-	}
-}
-
-// Syncsemacquire waits for a pairing Syncsemrelease on the same semaphore s.
-func runtime_Syncsemacquire(s *SyncSema) {
-	SemaWaiter w, *wake;
-	int64 t0;
-
-	w.g = g;
-	w.nrelease = -1;
-	w.next = nil;
-	w.releasetime = 0;
-	t0 = 0;
-	if(runtime·blockprofilerate > 0) {
-		t0 = runtime·cputicks();
-		w.releasetime = -1;
-	}
-
-	runtime·lock(&s->lock);
-	if(s->head && s->head->nrelease > 0) {
-		// have pending release, consume it
-		wake = nil;
-		s->head->nrelease--;
-		if(s->head->nrelease == 0) {
-			wake = s->head;
-			s->head = wake->next;
-			if(s->head == nil)
-				s->tail = nil;
-		}
-		runtime·unlock(&s->lock);
-		if(wake)
-			runtime·ready(wake->g);
-	} else {
-		// enqueue itself
-		if(s->tail == nil)
-			s->head = &w;
-		else
-			s->tail->next = &w;
-		s->tail = &w;
-		runtime·parkunlock(&s->lock, runtime·gostringnocopy((byte*)"semacquire"));
-		if(t0)
-			runtime·blockevent(w.releasetime - t0, 2);
-	}
-}
-
-// Syncsemrelease waits for n pairing Syncsemacquire on the same semaphore s.
-func runtime_Syncsemrelease(s *SyncSema, n uint32) {
-	SemaWaiter w, *wake;
-
-	w.g = g;
-	w.nrelease = (int32)n;
-	w.next = nil;
-	w.releasetime = 0;
-
-	runtime·lock(&s->lock);
-	while(w.nrelease > 0 && s->head && s->head->nrelease < 0) {
-		// have pending acquire, satisfy it
-		wake = s->head;
-		s->head = wake->next;
-		if(s->head == nil)
-			s->tail = nil;
-		if(wake->releasetime)
-			wake->releasetime = runtime·cputicks();
-		runtime·ready(wake->g);
-		w.nrelease--;
-	}
-	if(w.nrelease > 0) {
-		// enqueue itself
-		if(s->tail == nil)
-			s->head = &w;
-		else
-			s->tail->next = &w;
-		s->tail = &w;
-		runtime·parkunlock(&s->lock, runtime·gostringnocopy((byte*)"semarelease"));
-	} else
-		runtime·unlock(&s->lock);
-}

src/pkg/runtime/thunk.s

+// Copyright 2014 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.
+
+// This file exposes various internal runtime functions to other packages in std lib.
+
+#include "zasm_GOOS_GOARCH.h"
+#include "../../cmd/ld/textflag.h"
+
+#ifdef GOARCH_arm
+#define JMP B
+#endif
+
+TEXT sync·runtime_Syncsemacquire(SB),NOSPLIT,$0-0
+	JMP	runtime·syncsemacquire(SB)
+
+TEXT sync·runtime_Syncsemrelease(SB),NOSPLIT,$0-0
+	JMP	runtime·syncsemrelease(SB)
+
+TEXT sync·runtime_Syncsemcheck(SB),NOSPLIT,$0-0
+	JMP	runtime·syncsemcheck(SB)

src/pkg/sync/runtime.go

@@ -19,8 +19,12 @@ func runtime_Semacquire(s *uint32)
 // library and should not be used directly.
 func runtime_Semrelease(s *uint32)
 
-// Opaque representation of SyncSema in runtime/sema.goc.
-type syncSema [3]uintptr
+// Approximation of syncSema in runtime/sema.go.
+type syncSema struct {
+	lock uintptr
+	head unsafe.Pointer
+	tail unsafe.Pointer
+}
 
 // Syncsemacquire waits for a pairing Syncsemrelease on the same semaphore s.
 func runtime_Syncsemacquire(s *syncSema)