sync: make Cond MT-safe

This actually simplifies the code and avoids a heap allocation in the
call to Wait. Instead, it uses the Data field of the task to store
information on whether a task was signalled early.

src/sync/cond.go

@@ -1,19 +1,26 @@
 package sync
 
-import "internal/task"
+import (
+	"internal/task"
+	"unsafe"
+)
+
+// Condition variable.
+// A goroutine that called Wait() can be in one of a few states depending on the
+// Task.Data field:
+// - When entering Wait, and before going to sleep, the data field is 0.
+// - When the goroutine that calls Wait changes its data value from 0 to 1, it
+//   is going to sleep. It has not been awoken early.
+// - When instead a call to Signal or Broadcast can change the data field from 0
+//   to 1, it will _not_ go to sleep but be signalled early.
+//   This can happen when a concurrent call to Signal happens, or the Unlock
+//   function calls Signal for some reason.
 
 type Cond struct {
 	L Locker
 
-	unlocking *earlySignal
-	blocked   task.Stack
-}
-
-// earlySignal is a type used to implement a stack for signalling waiters while they are unlocking.
-type earlySignal struct {
-	next *earlySignal
-
-	signaled bool
+	blocked task.Stack
+	lock    task.PMutex
 }
 
 func NewCond(l Locker) *Cond {
@@ -24,14 +31,14 @@ func (c *Cond) trySignal() bool {
 	// Pop a blocked task off of the stack, and schedule it if applicable.
 	t := c.blocked.Pop()
 	if t != nil {
-		scheduleTask(t)
-		return true
-	}
+		dataPtr := (*task.Uint32)(unsafe.Pointer(&t.Data))
 
-	// If there any tasks which are currently unlocking, signal one.
-	if c.unlocking != nil {
-		c.unlocking.signaled = true
-		c.unlocking = c.unlocking.next
+		// The data value is 0 when the task is not yet sleeping, and 1 when it is.
+		if dataPtr.Swap(1) != 0 {
+			// The value was already 1, so the task went to sleep (or is about to go
+			// to sleep). Schedule the task to be resumed.
+			scheduleTask(t)
+		}
 		return true
 	}
 
@@ -40,21 +47,29 @@ func (c *Cond) trySignal() bool {
 }
 
 func (c *Cond) Signal() {
+	c.lock.Lock()
 	c.trySignal()
+	c.lock.Unlock()
 }
 
 func (c *Cond) Broadcast() {
 	// Signal everything.
+	c.lock.Lock()
 	for c.trySignal() {
 	}
+	c.lock.Unlock()
 }
 
 func (c *Cond) Wait() {
-	// Add an earlySignal frame to the stack so we can be signalled while unlocking.
-	early := earlySignal{
-		next: c.unlocking,
-	}
-	c.unlocking = &early
+	// Mark us as not yet signalled or sleeping.
+	t := task.Current()
+	dataPtr := (*task.Uint32)(unsafe.Pointer(&t.Data))
+	dataPtr.Store(0)
+
+	// Add us to the list of waiting goroutines.
+	c.lock.Lock()
+	c.blocked.Push(t)
+	c.lock.Unlock()
 
 	// Temporarily unlock L.
 	c.L.Unlock()
@@ -63,22 +78,14 @@ func (c *Cond) Wait() {
 	defer c.L.Lock()
 
 	// If we were signaled while unlocking, immediately complete.
-	if early.signaled {
+	if dataPtr.Swap(1) != 0 {
+		// The data value was already 1, so we got a signal already (and weren't
+		// scheduled because trySignal was the first to change the value).
 		return
 	}
 
-	// Remove the earlySignal frame.
-	prev := c.unlocking
-	for prev != nil && prev.next != &early {
-		prev = prev.next
-	}
-	if prev != nil {
-		prev.next = early.next
-	} else {
-		c.unlocking = early.next
-	}
-
-	// Wait for a signal.
-	c.blocked.Push(task.Current())
+	// We were the first to change the value from 0 to 1, meaning we did not get
+	// a signal during the call to Unlock(). So we wait until we do get a
+	// signal.
 	task.Pause()
 }