src/runtime/preempt.go - go - Git at Google

 // 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)
 	}
 }

 // canPreemptM reports whether mp is in a state that is safe to preempt.
 //
 // It is nosplit because it has nosplit callers.
 //
 //go:nosplit
 func canPreemptM(mp *m) bool {
 	return mp.locks == 0 && mp.mallocing == 0 && mp.preemptoff == "" && mp.p.ptr().status == _Prunning
 }
	// 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)
	}
	}

	// canPreemptM reports whether mp is in a state that is safe to preempt.
	//
	// It is nosplit because it has nosplit callers.
	//
	//go:nosplit
	func canPreemptM(mp *m) bool {
	return mp.locks == 0 && mp.mallocing == 0 && mp.preemptoff == "" && mp.p.ptr().status == _Prunning
	}