runtime: make channels parallelism-safe

Author: aykevl

src/internal/task/task.go

@@ -27,7 +27,7 @@ type Task struct {
 }
 
 // DataUint32 returns the Data field as a uint32. The value is only valid after
-// setting it through SetDataUint32.
+// setting it through SetDataUint32 or by storing to it using DataAtomicUint32.
 func (t *Task) DataUint32() uint32 {
 	return *(*uint32)(unsafe.Pointer(&t.Data))
 }
@@ -38,6 +38,11 @@ func (t *Task) SetDataUint32(val uint32) {
 	*(*uint32)(unsafe.Pointer(&t.Data)) = val
 }
 
+// DataAtomicUint32 returns the Data field as an atomic-if-needed Uint32 value.
+func (t *Task) DataAtomicUint32() *Uint32 {
+	return (*Uint32)(unsafe.Pointer(&t.Data))
+}
+
 // getGoroutineStackSize is a compiler intrinsic that returns the stack size for
 // the given function and falls back to the default stack size. It is replaced
 // with a load from a special section just before codegen.

src/runtime/chan.go

@@ -30,11 +30,12 @@ package runtime
 //   non-select operations) so that the select operation knows which case did
 //   proceed.
 //   The value is at the same time also a way that goroutines can be the first
-//   (and only) goroutine to 'take' a channel operation to change it from
-//   'waiting' to any other value. This is important for the select statement
-//   because multiple goroutines could try to let different channels in the
-//   select statement proceed at the same time. By using Task.Data, only a
-//   single channel operation in the select statement can proceed.
+//   (and only) goroutine to 'take' a channel operation using an atomic CAS
+//   operation to change it from 'waiting' to any other value. This is important
+//   for the select statement because multiple goroutines could try to let
+//   different channels in the select statement proceed at the same time. By
+//   using Task.Data, only a single channel operation in the select statement
+//   can proceed.
 // - It is possible for the channel queues to contain already-processed senders
 //   or receivers. This can happen when the select statement managed to proceed
 //   but the goroutine doing the select has not yet cleaned up the stale queue
@@ -49,15 +50,17 @@ import (
 
 // The runtime implementation of the Go 'chan' type.
 type channel struct {
-	closed      bool
-	elementSize uintptr
-	bufCap      uintptr // 'cap'
-	bufLen      uintptr // 'len'
-	bufHead     uintptr
-	bufTail     uintptr
-	senders     chanQueue
-	receivers   chanQueue
-	buf         unsafe.Pointer
+	closed       bool
+	selectLocked bool
+	elementSize  uintptr
+	bufCap       uintptr // 'cap'
+	bufLen       uintptr // 'len'
+	bufHead      uintptr
+	bufTail      uintptr
+	senders      chanQueue
+	receivers    chanQueue
+	lock         task.PMutex
+	buf          unsafe.Pointer
 }
 
 const (
@@ -73,7 +76,8 @@ type chanQueue struct {
 
 // Pus the next channel operation to the queue. All appropriate fields must have
 // been initialized already.
-// This function must be called with interrupts disabled.
+// This function must be called with interrupts disabled and the channel lock
+// held.
 func (q *chanQueue) push(node *channelOp) {
 	node.next = q.first
 	q.first = node
@@ -99,16 +103,17 @@ func (q *chanQueue) pop(chanOp uint32) *channelOp {
 		newDataValue := chanOp | popped.index<<2
 
 		// Try to be the first to proceed with this goroutine.
-		if popped.task.DataUint32() == chanOperationWaiting {
-			popped.task.SetDataUint32(newDataValue)
+		swapped := popped.task.DataAtomicUint32().CompareAndSwap(0, newDataValue)
+		if swapped {
 			return popped
 		}
 	}
 }
 
 // Remove the given to-be-removed node from the queue if it is part of the
 // queue. If there are multiple, only one will be removed.
-// This function must be called with interrupts disabled.
+// This function must be called with interrupts disabled and the channel lock
+// held.
 func (q *chanQueue) remove(remove *channelOp) {
 	n := &q.first
 	for *n != nil {
@@ -159,8 +164,8 @@ func chanCap(c *channel) int {
 }
 
 // Push the value to the channel buffer array, for a send operation.
-// This function may only be called when interrupts are disabled and it is known
-// there is space available in the buffer.
+// This function may only be called when interrupts are disabled, the channel is
+// locked and it is known there is space available in the buffer.
 func (ch *channel) bufferPush(value unsafe.Pointer) {
 	elemAddr := unsafe.Add(ch.buf, ch.bufHead*ch.elementSize)
 	ch.bufLen++
@@ -174,8 +179,8 @@ func (ch *channel) bufferPush(value unsafe.Pointer) {
 
 // Pop a value from the channel buffer and store it in the 'value' pointer, for
 // a receive operation.
-// This function may only be called when interrupts are disabled and it is known
-// there is at least one value available in the buffer.
+// This function may only be called when interrupts are disabled, the channel is
+// locked and it is known there is at least one value available in the buffer.
 func (ch *channel) bufferPop(value unsafe.Pointer) {
 	elemAddr := unsafe.Add(ch.buf, ch.bufTail*ch.elementSize)
 	ch.bufLen--
@@ -191,7 +196,8 @@ func (ch *channel) bufferPop(value unsafe.Pointer) {
 }
 
 // Try to proceed with this send operation without blocking, and return whether
-// the send succeeded. Interrupts must be disabled when calling this function.
+// the send succeeded. Interrupts must be disabled and the lock must be held
+// when calling this function.
 func (ch *channel) trySend(value unsafe.Pointer) bool {
 	// To make sure we send values in the correct order, we can only send
 	// directly to a receiver when there are no values in the buffer.
@@ -230,9 +236,11 @@ func chanSend(ch *channel, value unsafe.Pointer, op *channelOp) {
 	}
 
 	mask := interrupt.Disable()
+	ch.lock.Lock()
 
 	// See whether we can proceed immediately, and if so, return early.
 	if ch.trySend(value) {
+		ch.lock.Unlock()
 		interrupt.Restore(mask)
 		return
 	}
@@ -244,9 +252,12 @@ func chanSend(ch *channel, value unsafe.Pointer, op *channelOp) {
 	op.index = 0
 	op.value = value
 	ch.senders.push(op)
+	ch.lock.Unlock()
 	interrupt.Restore(mask)
 
 	// Wait until this goroutine is resumed.
+	// It might be resumed after Unlock() and before Pause(). In that case,
+	// because we use semaphores, the Pause() will continue immediately.
 	task.Pause()
 
 	// Check whether the sent happened normally (not because the channel was
@@ -258,8 +269,8 @@ func chanSend(ch *channel, value unsafe.Pointer, op *channelOp) {
 }
 
 // Try to proceed with this receive operation without blocking, and return
-// whether the receive operation succeeded. Interrupts must be disabled when
-// calling this function.
+// whether the receive operation succeeded. Interrupts must be disabled and the
+// lock must be held when calling this function.
 func (ch *channel) tryRecv(value unsafe.Pointer) (received, ok bool) {
 	// To make sure we keep the values in the channel in the correct order, we
 	// first have to read values from the buffer before we can look at the
@@ -303,8 +314,10 @@ func chanRecv(ch *channel, value unsafe.Pointer, op *channelOp) bool {
 	}
 
 	mask := interrupt.Disable()
+	ch.lock.Lock()
 
 	if received, ok := ch.tryRecv(value); received {
+		ch.lock.Unlock()
 		interrupt.Restore(mask)
 		return ok
 	}
@@ -317,6 +330,7 @@ func chanRecv(ch *channel, value unsafe.Pointer, op *channelOp) bool {
 	op.task = t
 	op.index = 0
 	ch.receivers.push(op)
+	ch.lock.Unlock()
 	interrupt.Restore(mask)
 
 	// Wait until the goroutine is resumed.
@@ -335,9 +349,11 @@ func chanClose(ch *channel) {
 	}
 
 	mask := interrupt.Disable()
+	ch.lock.Lock()
 
 	if ch.closed {
 		// Not allowed by the language spec.
+		ch.lock.Unlock()
 		interrupt.Restore(mask)
 		runtimePanic("close of closed channel")
 	}
@@ -370,14 +386,56 @@ func chanClose(ch *channel) {
 
 	ch.closed = true
 
+	ch.lock.Unlock()
 	interrupt.Restore(mask)
 }
 
+// We currently use a global select lock to avoid deadlocks while locking each
+// individual channel in the select. Without this global lock, two select
+// operations that have a different order of the same channels could end up in a
+// deadlock. This global lock is inefficient if there are many select operations
+// happening in parallel, but gets the job done.
+//
+// If this becomes a performance issue, we can see how the Go runtime does this.
+// I think it does this by sorting all states by channel address and then
+// locking them in that order to avoid this deadlock.
+var chanSelectLock task.PMutex
+
+// Lock all channels (taking care to skip duplicate channels).
+func lockAllStates(states []chanSelectState) {
+	if !hasParallelism {
+		return
+	}
+	for _, state := range states {
+		if state.ch != nil && !state.ch.selectLocked {
+			state.ch.lock.Lock()
+			state.ch.selectLocked = true
+		}
+	}
+}
+
+// Unlock all channels (taking care to skip duplicate channels).
+func unlockAllStates(states []chanSelectState) {
+	if !hasParallelism {
+		return
+	}
+	for _, state := range states {
+		if state.ch != nil && state.ch.selectLocked {
+			state.ch.lock.Unlock()
+			state.ch.selectLocked = false
+		}
+	}
+}
+
 // chanSelect implements blocking or non-blocking select operations.
 // The 'ops' slice must be set if (and only if) this is a blocking select.
 func chanSelect(recvbuf unsafe.Pointer, states []chanSelectState, ops []channelOp) (uint32, bool) {
 	mask := interrupt.Disable()
 
+	// Lock everything.
+	chanSelectLock.Lock()
+	lockAllStates(states)
+
 	const selectNoIndex = ^uint32(0)
 	selectIndex := selectNoIndex
 	selectOk := true
@@ -409,6 +467,8 @@ func chanSelect(recvbuf unsafe.Pointer, states []chanSelectState, ops []channelO
 	// return early.
 	blocking := len(ops) != 0
 	if selectIndex != selectNoIndex || !blocking {
+		unlockAllStates(states)
+		chanSelectLock.Unlock()
 		interrupt.Restore(mask)
 		return selectIndex, selectOk
 	}
@@ -417,8 +477,8 @@ func chanSelect(recvbuf unsafe.Pointer, states []chanSelectState, ops []channelO
 	// become more complicated.
 	// We add ourselves as a sender/receiver to every channel, and wait for the
 	// first one to complete. Only one will successfully complete, because
-	// senders and receivers will check t.Data for the state so that only one
-	// will be able to "take" this select operation.
+	// senders and receivers use a compare-and-exchange atomic operation on
+	// t.Data so that only one will be able to "take" this select operation.
 	t := task.Current()
 	t.Ptr = recvbuf
 	t.SetDataUint32(chanOperationWaiting)
@@ -438,13 +498,17 @@ func chanSelect(recvbuf unsafe.Pointer, states []chanSelectState, ops []channelO
 	}
 
 	// Now we wait until one of the send/receive operations can proceed.
+	unlockAllStates(states)
+	chanSelectLock.Unlock()
 	interrupt.Restore(mask)
 	task.Pause()
 
 	// Resumed, so one channel operation must have progressed.
 
 	// Make sure all channel ops are removed from the senders/receivers
 	// queue before we return and the memory of them becomes invalid.
+	chanSelectLock.Lock()
+	lockAllStates(states)
 	for i, state := range states {
 		if state.ch == nil {
 			continue
@@ -458,6 +522,8 @@ func chanSelect(recvbuf unsafe.Pointer, states []chanSelectState, ops []channelO
 		}
 		interrupt.Restore(mask)
 	}
+	unlockAllStates(states)
+	chanSelectLock.Unlock()
 
 	// Pull the return values out of t.Data (which contains two bitfields).
 	selectIndex = t.DataUint32() >> 2

src/runtime/scheduler_cooperative.go

@@ -21,6 +21,12 @@ import (
 // queue a new scheduler invocation using setTimeout.
 const asyncScheduler = GOOS == "js"
 
+const hasScheduler = true
+
+// Concurrency is not parallelism. While the cooperative scheduler has
+// concurrency, it does not have parallelism.
+const hasParallelism = false
+
 // Queues used by the scheduler.
 var (
 	runqueue           task.Queue
@@ -248,5 +254,3 @@ func run() {
 	}()
 	scheduler(false)
 }
-
-const hasScheduler = true

src/runtime/scheduler_none.go

@@ -6,6 +6,9 @@ import "internal/task"
 
 const hasScheduler = false
 
+// No goroutines are allowed, so there's no parallelism anywhere.
+const hasParallelism = false
+
 // run is called by the program entry point to execute the go program.
 // With the "none" scheduler, init and the main function are invoked directly.
 func run() {