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diff --git a/src/runtime/preempt.go b/src/runtime/preempt.go
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+++ b/src/runtime/preempt.go
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+// Copyright 2019 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.
+
+// Goroutine preemption
+//
+// A goroutine can be preempted at any safe-point. Currently, there
+// are a few categories of safe-points:
+//
+// 1. A blocked safe-point occurs for the duration that a goroutine is
+//    descheduled, blocked on synchronization, or in a system call.
+//
+// 2. Synchronous safe-points occur when a running goroutine checks
+//    for a preemption request.
+//
+// At both blocked and synchronous safe-points, a goroutine's CPU
+// state is minimal and the garbage collector has complete information
+// about its entire stack. This makes it possible to deschedule a
+// goroutine with minimal space, and to precisely scan a goroutine's
+// stack.
+//
+// Synchronous safe-points are implemented by overloading the stack
+// bound check in function prologues. To preempt a goroutine at the
+// next synchronous safe-point, the runtime poisons the goroutine's
+// stack bound to a value that will cause the next stack bound check
+// to fail and enter the stack growth implementation, which will
+// detect that it was actually a preemption and redirect to preemption
+// handling.
+
+package runtime
+
+type suspendGState struct {
+	g *g
+
+	// dead indicates the goroutine was not suspended because it
+	// is dead. This goroutine could be reused after the dead
+	// state was observed, so the caller must not assume that it
+	// remains dead.
+	dead bool
+
+	// stopped indicates that this suspendG transitioned the G to
+	// _Gwaiting via g.preemptStop and thus is responsible for
+	// readying it when done.
+	stopped bool
+}
+
+// suspendG suspends goroutine gp at a safe-point and returns the
+// state of the suspended goroutine. The caller gets read access to
+// the goroutine until it calls resumeG.
+//
+// It is safe for multiple callers to attempt to suspend the same
+// goroutine at the same time. The goroutine may execute between
+// subsequent successful suspend operations. The current
+// implementation grants exclusive access to the goroutine, and hence
+// multiple callers will serialize. However, the intent is to grant
+// shared read access, so please don't depend on exclusive access.
+//
+// This must be called from the system stack and the user goroutine on
+// the current M (if any) must be in a preemptible state. This
+// prevents deadlocks where two goroutines attempt to suspend each
+// other and both are in non-preemptible states. There are other ways
+// to resolve this deadlock, but this seems simplest.
+//
+// TODO(austin): What if we instead required this to be called from a
+// user goroutine? Then we could deschedule the goroutine while
+// waiting instead of blocking the thread. If two goroutines tried to
+// suspend each other, one of them would win and the other wouldn't
+// complete the suspend until it was resumed. We would have to be
+// careful that they couldn't actually queue up suspend for each other
+// and then both be suspended. This would also avoid the need for a
+// kernel context switch in the synchronous case because we could just
+// directly schedule the waiter. The context switch is unavoidable in
+// the signal case.
+//
+//go:systemstack
+func suspendG(gp *g) suspendGState {
+	if mp := getg().m; mp.curg != nil && readgstatus(mp.curg) == _Grunning {
+		// Since we're on the system stack of this M, the user
+		// G is stuck at an unsafe point. If another goroutine
+		// were to try to preempt m.curg, it could deadlock.
+		throw("suspendG from non-preemptible goroutine")
+	}
+
+	// See https://golang.org/cl/21503 for justification of the yield delay.
+	const yieldDelay = 10 * 1000
+	var nextYield int64
+
+	// Drive the goroutine to a preemption point.
+	stopped := false
+	for i := 0; ; i++ {
+		switch s := readgstatus(gp); s {
+		default:
+			if s&_Gscan != 0 {
+				// Someone else is suspending it. Wait
+				// for them to finish.
+				//
+				// TODO: It would be nicer if we could
+				// coalesce suspends.
+				break
+			}
+
+			dumpgstatus(gp)
+			throw("invalid g status")
+
+		case _Gdead:
+			// Nothing to suspend.
+			//
+			// preemptStop may need to be cleared, but
+			// doing that here could race with goroutine
+			// reuse. Instead, goexit0 clears it.
+			return suspendGState{dead: true}
+
+		case _Gcopystack:
+			// The stack is being copied. We need to wait
+			// until this is done.
+
+		case _Gpreempted:
+			// We (or someone else) suspended the G. Claim
+			// ownership of it by transitioning it to
+			// _Gwaiting.
+			if !casGFromPreempted(gp, _Gpreempted, _Gwaiting) {
+				break
+			}
+
+			// We stopped the G, so we have to ready it later.
+			stopped = true
+
+			s = _Gwaiting
+			fallthrough
+
+		case _Grunnable, _Gsyscall, _Gwaiting:
+			// Claim goroutine by setting scan bit.
+			// This may race with execution or readying of gp.
+			// The scan bit keeps it from transition state.
+			if !castogscanstatus(gp, s, s|_Gscan) {
+				break
+			}
+
+			// Clear the preemption request. It's safe to
+			// reset the stack guard because we hold the
+			// _Gscan bit and thus own the stack.
+			gp.preemptStop = false
+			gp.preempt = false
+			gp.stackguard0 = gp.stack.lo + _StackGuard
+
+			// The goroutine was already at a safe-point
+			// and we've now locked that in.
+			//
+			// TODO: It would be much better if we didn't
+			// leave it in _Gscan, but instead gently
+			// prevented its scheduling until resumption.
+			// Maybe we only use this to bump a suspended
+			// count and the scheduler skips suspended
+			// goroutines? That wouldn't be enough for
+			// {_Gsyscall,_Gwaiting} -> _Grunning. Maybe
+			// for all those transitions we need to check
+			// suspended and deschedule?
+			return suspendGState{g: gp, stopped: stopped}
+
+		case _Grunning:
+			// Optimization: if there is already a pending preemption request
+			// (from the previous loop iteration), don't bother with the atomics.
+			if gp.preemptStop && gp.preempt && gp.stackguard0 == stackPreempt {
+				break
+			}
+
+			// Temporarily block state transitions.
+			if !castogscanstatus(gp, _Grunning, _Gscanrunning) {
+				break
+			}
+
+			// Request synchronous preemption.
+			gp.preemptStop = true
+			gp.preempt = true
+			gp.stackguard0 = stackPreempt
+
+			// TODO: Inject asynchronous preemption.
+
+			casfrom_Gscanstatus(gp, _Gscanrunning, _Grunning)
+		}
+
+		// TODO: Don't busy wait. This loop should really only
+		// be a simple read/decide/CAS loop that only fails if
+		// there's an active race. Once the CAS succeeds, we
+		// should queue up the preemption (which will require
+		// it to be reliable in the _Grunning case, not
+		// best-effort) and then sleep until we're notified
+		// that the goroutine is suspended.
+		if i == 0 {
+			nextYield = nanotime() + yieldDelay
+		}
+		if nanotime() < nextYield {
+			procyield(10)
+		} else {
+			osyield()
+			nextYield = nanotime() + yieldDelay/2
+		}
+	}
+}
+
+// resumeG undoes the effects of suspendG, allowing the suspended
+// goroutine to continue from its current safe-point.
+func resumeG(state suspendGState) {
+	if state.dead {
+		// We didn't actually stop anything.
+		return
+	}
+
+	gp := state.g
+	switch s := readgstatus(gp); s {
+	default:
+		dumpgstatus(gp)
+		throw("unexpected g status")
+
+	case _Grunnable | _Gscan,
+		_Gwaiting | _Gscan,
+		_Gsyscall | _Gscan:
+		casfrom_Gscanstatus(gp, s, s&^_Gscan)
+	}
+
+	if state.stopped {
+		// We stopped it, so we need to re-schedule it.
+		ready(gp, 0, true)
+	}
+}