Adding upstream version 1.19.8.upstream/1.19.8upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 453 insertions, 0 deletions

diff --git a/src/runtime/preempt.go b/src/runtime/preempt.go
new file mode 100644
index 0000000..da24f50
--- /dev/null
+++ b/src/runtime/preempt.go
@@ -0,0 +1,453 @@
+// 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.
+//
+// 3. Asynchronous safe-points occur at any instruction in user code
+//    where the goroutine can be safely paused and a conservative
+//    stack and register scan can find stack roots. The runtime can
+//    stop a goroutine at an async safe-point using a signal.
+//
+// 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.
+//
+// Preemption at asynchronous safe-points is implemented by suspending
+// the thread using an OS mechanism (e.g., signals) and inspecting its
+// state to determine if the goroutine was at an asynchronous
+// safe-point. Since the thread suspension itself is generally
+// asynchronous, it also checks if the running goroutine wants to be
+// preempted, since this could have changed. If all conditions are
+// satisfied, it adjusts the signal context to make it look like the
+// signaled thread just called asyncPreempt and resumes the thread.
+// asyncPreempt spills all registers and enters the scheduler.
+//
+// (An alternative would be to preempt in the signal handler itself.
+// This would let the OS save and restore the register state and the
+// runtime would only need to know how to extract potentially
+// pointer-containing registers from the signal context. However, this
+// would consume an M for every preempted G, and the scheduler itself
+// is not designed to run from a signal handler, as it tends to
+// allocate memory and start threads in the preemption path.)
+
+package runtime
+
+import (
+	"internal/abi"
+	"internal/goarch"
+	"runtime/internal/atomic"
+)
+
+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
+	var asyncM *m
+	var asyncGen uint32
+	var nextPreemptM int64
+	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 && asyncM == gp.m && atomic.Load(&asyncM.preemptGen) == asyncGen {
+				break
+			}
+
+			// Temporarily block state transitions.
+			if !castogscanstatus(gp, _Grunning, _Gscanrunning) {
+				break
+			}
+
+			// Request synchronous preemption.
+			gp.preemptStop = true
+			gp.preempt = true
+			gp.stackguard0 = stackPreempt
+
+			// Prepare for asynchronous preemption.
+			asyncM2 := gp.m
+			asyncGen2 := atomic.Load(&asyncM2.preemptGen)
+			needAsync := asyncM != asyncM2 || asyncGen != asyncGen2
+			asyncM = asyncM2
+			asyncGen = asyncGen2
+
+			casfrom_Gscanstatus(gp, _Gscanrunning, _Grunning)
+
+			// Send asynchronous preemption. We do this
+			// after CASing the G back to _Grunning
+			// because preemptM may be synchronous and we
+			// don't want to catch the G just spinning on
+			// its status.
+			if preemptMSupported && debug.asyncpreemptoff == 0 && needAsync {
+				// Rate limit preemptM calls. This is
+				// particularly important on Windows
+				// where preemptM is actually
+				// synchronous and the spin loop here
+				// can lead to live-lock.
+				now := nanotime()
+				if now >= nextPreemptM {
+					nextPreemptM = now + yieldDelay/2
+					preemptM(asyncM)
+				}
+			}
+		}
+
+		// 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
+}
+
+//go:generate go run mkpreempt.go
+
+// asyncPreempt saves all user registers and calls asyncPreempt2.
+//
+// When stack scanning encounters an asyncPreempt frame, it scans that
+// frame and its parent frame conservatively.
+//
+// asyncPreempt is implemented in assembly.
+func asyncPreempt()
+
+//go:nosplit
+func asyncPreempt2() {
+	gp := getg()
+	gp.asyncSafePoint = true
+	if gp.preemptStop {
+		mcall(preemptPark)
+	} else {
+		mcall(gopreempt_m)
+	}
+	gp.asyncSafePoint = false
+}
+
+// asyncPreemptStack is the bytes of stack space required to inject an
+// asyncPreempt call.
+var asyncPreemptStack = ^uintptr(0)
+
+func init() {
+	f := findfunc(abi.FuncPCABI0(asyncPreempt))
+	total := funcMaxSPDelta(f)
+	f = findfunc(abi.FuncPCABIInternal(asyncPreempt2))
+	total += funcMaxSPDelta(f)
+	// Add some overhead for return PCs, etc.
+	asyncPreemptStack = uintptr(total) + 8*goarch.PtrSize
+	if asyncPreemptStack > _StackLimit {
+		// We need more than the nosplit limit. This isn't
+		// unsafe, but it may limit asynchronous preemption.
+		//
+		// This may be a problem if we start using more
+		// registers. In that case, we should store registers
+		// in a context object. If we pre-allocate one per P,
+		// asyncPreempt can spill just a few registers to the
+		// stack, then grab its context object and spill into
+		// it. When it enters the runtime, it would allocate a
+		// new context for the P.
+		print("runtime: asyncPreemptStack=", asyncPreemptStack, "\n")
+		throw("async stack too large")
+	}
+}
+
+// wantAsyncPreempt returns whether an asynchronous preemption is
+// queued for gp.
+func wantAsyncPreempt(gp *g) bool {
+	// Check both the G and the P.
+	return (gp.preempt || gp.m.p != 0 && gp.m.p.ptr().preempt) && readgstatus(gp)&^_Gscan == _Grunning
+}
+
+// isAsyncSafePoint reports whether gp at instruction PC is an
+// asynchronous safe point. This indicates that:
+//
+// 1. It's safe to suspend gp and conservatively scan its stack and
+// registers. There are no potentially hidden pointer values and it's
+// not in the middle of an atomic sequence like a write barrier.
+//
+// 2. gp has enough stack space to inject the asyncPreempt call.
+//
+// 3. It's generally safe to interact with the runtime, even if we're
+// in a signal handler stopped here. For example, there are no runtime
+// locks held, so acquiring a runtime lock won't self-deadlock.
+//
+// In some cases the PC is safe for asynchronous preemption but it
+// also needs to adjust the resumption PC. The new PC is returned in
+// the second result.
+func isAsyncSafePoint(gp *g, pc, sp, lr uintptr) (bool, uintptr) {
+	mp := gp.m
+
+	// Only user Gs can have safe-points. We check this first
+	// because it's extremely common that we'll catch mp in the
+	// scheduler processing this G preemption.
+	if mp.curg != gp {
+		return false, 0
+	}
+
+	// Check M state.
+	if mp.p == 0 || !canPreemptM(mp) {
+		return false, 0
+	}
+
+	// Check stack space.
+	if sp < gp.stack.lo || sp-gp.stack.lo < asyncPreemptStack {
+		return false, 0
+	}
+
+	// Check if PC is an unsafe-point.
+	f := findfunc(pc)
+	if !f.valid() {
+		// Not Go code.
+		return false, 0
+	}
+	if (GOARCH == "mips" || GOARCH == "mipsle" || GOARCH == "mips64" || GOARCH == "mips64le") && lr == pc+8 && funcspdelta(f, pc, nil) == 0 {
+		// We probably stopped at a half-executed CALL instruction,
+		// where the LR is updated but the PC has not. If we preempt
+		// here we'll see a seemingly self-recursive call, which is in
+		// fact not.
+		// This is normally ok, as we use the return address saved on
+		// stack for unwinding, not the LR value. But if this is a
+		// call to morestack, we haven't created the frame, and we'll
+		// use the LR for unwinding, which will be bad.
+		return false, 0
+	}
+	up, startpc := pcdatavalue2(f, _PCDATA_UnsafePoint, pc)
+	if up == _PCDATA_UnsafePointUnsafe {
+		// Unsafe-point marked by compiler. This includes
+		// atomic sequences (e.g., write barrier) and nosplit
+		// functions (except at calls).
+		return false, 0
+	}
+	if fd := funcdata(f, _FUNCDATA_LocalsPointerMaps); fd == nil || f.flag&funcFlag_ASM != 0 {
+		// This is assembly code. Don't assume it's well-formed.
+		// TODO: Empirically we still need the fd == nil check. Why?
+		//
+		// TODO: Are there cases that are safe but don't have a
+		// locals pointer map, like empty frame functions?
+		// It might be possible to preempt any assembly functions
+		// except the ones that have funcFlag_SPWRITE set in f.flag.
+		return false, 0
+	}
+	name := funcname(f)
+	if inldata := funcdata(f, _FUNCDATA_InlTree); inldata != nil {
+		inltree := (*[1 << 20]inlinedCall)(inldata)
+		ix := pcdatavalue(f, _PCDATA_InlTreeIndex, pc, nil)
+		if ix >= 0 {
+			name = funcnameFromNameoff(f, inltree[ix].func_)
+		}
+	}
+	if hasPrefix(name, "runtime.") ||
+		hasPrefix(name, "runtime/internal/") ||
+		hasPrefix(name, "reflect.") {
+		// For now we never async preempt the runtime or
+		// anything closely tied to the runtime. Known issues
+		// include: various points in the scheduler ("don't
+		// preempt between here and here"), much of the defer
+		// implementation (untyped info on stack), bulk write
+		// barriers (write barrier check),
+		// reflect.{makeFuncStub,methodValueCall}.
+		//
+		// TODO(austin): We should improve this, or opt things
+		// in incrementally.
+		return false, 0
+	}
+	switch up {
+	case _PCDATA_Restart1, _PCDATA_Restart2:
+		// Restartable instruction sequence. Back off PC to
+		// the start PC.
+		if startpc == 0 || startpc > pc || pc-startpc > 20 {
+			throw("bad restart PC")
+		}
+		return true, startpc
+	case _PCDATA_RestartAtEntry:
+		// Restart from the function entry at resumption.
+		return true, f.entry()
+	}
+	return true, pc
+}