[gostls13.git] /

// Copyright 2009 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.

package runtime

import "unsafe"

// The code in this file implements stack trace walking for all architectures.
// The most important fact about a given architecture is whether it uses a link register.
// On systems with link registers, the prologue for a non-leaf function stores the
// incoming value of LR at the bottom of the newly allocated stack frame.
// On systems without link registers, the architecture pushes a return PC during
// the call instruction, so the return PC ends up above the stack frame.
// In this file, the return PC is always called LR, no matter how it was found.
//
// To date, the opposite of a link register architecture is an x86 architecture.
// This code may need to change if some other kind of non-link-register
// architecture comes along.
//
// The other important fact is the size of a pointer: on 32-bit systems the LR
// takes up only 4 bytes on the stack, while on 64-bit systems it takes up 8 bytes.
// Typically this is ptrSize.
//
// As an exception, amd64p32 has ptrSize == 4 but the CALL instruction still
// stores an 8-byte return PC onto the stack. To accommodate this, we use regSize
// as the size of the architecture-pushed return PC.
//
// usesLR is defined below. ptrSize and regSize are defined in stubs.go.

const usesLR = GOARCH != "amd64" && GOARCH != "amd64p32" && GOARCH != "386"

var (
        // initialized in tracebackinit
        goexitPC             uintptr
        jmpdeferPC           uintptr
        mcallPC              uintptr
        morestackPC          uintptr
        mstartPC             uintptr
        rt0_goPC             uintptr
        sigpanicPC           uintptr
        runfinqPC            uintptr
        backgroundgcPC       uintptr
        bgsweepPC            uintptr
        forcegchelperPC      uintptr
        timerprocPC          uintptr
        gcBgMarkWorkerPC     uintptr
        systemstack_switchPC uintptr
        systemstackPC        uintptr
        stackBarrierPC       uintptr

        gogoPC uintptr

        externalthreadhandlerp uintptr // initialized elsewhere
)

func tracebackinit() {
        // Go variable initialization happens late during runtime startup.
        // Instead of initializing the variables above in the declarations,
        // schedinit calls this function so that the variables are
        // initialized and available earlier in the startup sequence.
        goexitPC = funcPC(goexit)
        jmpdeferPC = funcPC(jmpdefer)
        mcallPC = funcPC(mcall)
        morestackPC = funcPC(morestack)
        mstartPC = funcPC(mstart)
        rt0_goPC = funcPC(rt0_go)
        sigpanicPC = funcPC(sigpanic)
        runfinqPC = funcPC(runfinq)
        backgroundgcPC = funcPC(backgroundgc)
        bgsweepPC = funcPC(bgsweep)
        forcegchelperPC = funcPC(forcegchelper)
        timerprocPC = funcPC(timerproc)
        gcBgMarkWorkerPC = funcPC(gcBgMarkWorker)
        systemstack_switchPC = funcPC(systemstack_switch)
        systemstackPC = funcPC(systemstack)
        stackBarrierPC = funcPC(stackBarrier)

        // used by sigprof handler
        gogoPC = funcPC(gogo)
}

// Traceback over the deferred function calls.
// Report them like calls that have been invoked but not started executing yet.
func tracebackdefers(gp *g, callback func(*stkframe, unsafe.Pointer) bool, v unsafe.Pointer) {
        var frame stkframe
        for d := gp._defer; d != nil; d = d.link {
                fn := d.fn
                if fn == nil {
                        // Defer of nil function. Args don't matter.
                        frame.pc = 0
                        frame.fn = nil
                        frame.argp = 0
                        frame.arglen = 0
                        frame.argmap = nil
                } else {
                        frame.pc = uintptr(fn.fn)
                        f := findfunc(frame.pc)
                        if f == nil {
                                print("runtime: unknown pc in defer ", hex(frame.pc), "\n")
                                throw("unknown pc")
                        }
                        frame.fn = f
                        frame.argp = uintptr(deferArgs(d))
                        setArgInfo(&frame, f, true)
                }
                frame.continpc = frame.pc
                if !callback((*stkframe)(noescape(unsafe.Pointer(&frame))), v) {
                        return
                }
        }
}

// Generic traceback.  Handles runtime stack prints (pcbuf == nil),
// the runtime.Callers function (pcbuf != nil), as well as the garbage
// collector (callback != nil).  A little clunky to merge these, but avoids
// duplicating the code and all its subtlety.
func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max int, callback func(*stkframe, unsafe.Pointer) bool, v unsafe.Pointer, flags uint) int {
        if goexitPC == 0 {
                throw("gentraceback before goexitPC initialization")
        }
        g := getg()
        if g == gp && g == g.m.curg {
                // The starting sp has been passed in as a uintptr, and the caller may
                // have other uintptr-typed stack references as well.
                // If during one of the calls that got us here or during one of the
                // callbacks below the stack must be grown, all these uintptr references
                // to the stack will not be updated, and gentraceback will continue
                // to inspect the old stack memory, which may no longer be valid.
                // Even if all the variables were updated correctly, it is not clear that
                // we want to expose a traceback that begins on one stack and ends
                // on another stack. That could confuse callers quite a bit.
                // Instead, we require that gentraceback and any other function that
                // accepts an sp for the current goroutine (typically obtained by
                // calling getcallersp) must not run on that goroutine's stack but
                // instead on the g0 stack.
                throw("gentraceback cannot trace user goroutine on its own stack")
        }
        gotraceback := gotraceback(nil)

        // Fix up returns to the stack barrier by fetching the
        // original return PC from gp.stkbar.
        stkbar := gp.stkbar[gp.stkbarPos:]

        if pc0 == ^uintptr(0) && sp0 == ^uintptr(0) { // Signal to fetch saved values from gp.
                if gp.syscallsp != 0 {
                        pc0 = gp.syscallpc
                        sp0 = gp.syscallsp
                        if usesLR {
                                lr0 = 0
                        }
                } else {
                        pc0 = gp.sched.pc
                        sp0 = gp.sched.sp
                        if usesLR {
                                lr0 = gp.sched.lr
                        }
                }
        }

        nprint := 0
        var frame stkframe
        frame.pc = pc0
        frame.sp = sp0
        if usesLR {
                frame.lr = lr0
        }
        waspanic := false
        printing := pcbuf == nil && callback == nil
        _defer := gp._defer

        for _defer != nil && uintptr(_defer.sp) == _NoArgs {
                _defer = _defer.link
        }

        // If the PC is zero, it's likely a nil function call.
        // Start in the caller's frame.
        if frame.pc == 0 {
                if usesLR {
                        frame.pc = *(*uintptr)(unsafe.Pointer(frame.sp))
                        frame.lr = 0
                } else {
                        frame.pc = uintptr(*(*uintreg)(unsafe.Pointer(frame.sp)))
                        frame.sp += regSize
                }
        }

        f := findfunc(frame.pc)
        if f == nil {
                if callback != nil {
                        print("runtime: unknown pc ", hex(frame.pc), "\n")
                        throw("unknown pc")
                }
                return 0
        }
        frame.fn = f

        n := 0
        for n < max {
                // Typically:
                //      pc is the PC of the running function.
                //      sp is the stack pointer at that program counter.
                //      fp is the frame pointer (caller's stack pointer) at that program counter, or nil if unknown.
                //      stk is the stack containing sp.
                //      The caller's program counter is lr, unless lr is zero, in which case it is *(uintptr*)sp.
                f = frame.fn

                // Found an actual function.
                // Derive frame pointer and link register.
                if frame.fp == 0 {
                        // We want to jump over the systemstack switch. If we're running on the
                        // g0, this systemstack is at the top of the stack.
                        // if we're not on g0 or there's a no curg, then this is a regular call.
                        sp := frame.sp
                        if flags&_TraceJumpStack != 0 && f.entry == systemstackPC && gp == g.m.g0 && gp.m.curg != nil {
                                sp = gp.m.curg.sched.sp
                                stkbar = gp.m.curg.stkbar[gp.m.curg.stkbarPos:]
                        }
                        frame.fp = sp + uintptr(funcspdelta(f, frame.pc))
                        if !usesLR {
                                // On x86, call instruction pushes return PC before entering new function.
                                frame.fp += regSize
                        }
                }
                var flr *_func
                if topofstack(f) {
                        frame.lr = 0
                        flr = nil
                } else if usesLR && f.entry == jmpdeferPC {
                        // jmpdefer modifies SP/LR/PC non-atomically.
                        // If a profiling interrupt arrives during jmpdefer,
                        // the stack unwind may see a mismatched register set
                        // and get confused. Stop if we see PC within jmpdefer
                        // to avoid that confusion.
                        // See golang.org/issue/8153.
                        if callback != nil {
                                throw("traceback_arm: found jmpdefer when tracing with callback")
                        }
                        frame.lr = 0
                } else {
                        var lrPtr uintptr
                        if usesLR {
                                if n == 0 && frame.sp < frame.fp || frame.lr == 0 {
                                        lrPtr = frame.sp
                                        frame.lr = *(*uintptr)(unsafe.Pointer(lrPtr))
                                }
                        } else {
                                if frame.lr == 0 {
                                        lrPtr = frame.fp - regSize
                                        frame.lr = uintptr(*(*uintreg)(unsafe.Pointer(lrPtr)))
                                }
                        }
                        if frame.lr == stackBarrierPC {
                                // Recover original PC.
                                if stkbar[0].savedLRPtr != lrPtr {
                                        print("found next stack barrier at ", hex(lrPtr), "; expected ")
                                        gcPrintStkbars(stkbar)
                                        print("\n")
                                        throw("missed stack barrier")
                                }
                                frame.lr = stkbar[0].savedLRVal
                                stkbar = stkbar[1:]
                        }
                        flr = findfunc(frame.lr)
                        if flr == nil {
                                // This happens if you get a profiling interrupt at just the wrong time.
                                // In that context it is okay to stop early.
                                // But if callback is set, we're doing a garbage collection and must
                                // get everything, so crash loudly.
                                if callback != nil {
                                        print("runtime: unexpected return pc for ", funcname(f), " called from ", hex(frame.lr), "\n")
                                        throw("unknown caller pc")
                                }
                        }
                }

                frame.varp = frame.fp
                if !usesLR {
                        // On x86, call instruction pushes return PC before entering new function.
                        frame.varp -= regSize
                }

                // If framepointer_enabled and there's a frame, then
                // there's a saved bp here.
                if framepointer_enabled && GOARCH == "amd64" && frame.varp > frame.sp {
                        frame.varp -= regSize
                }

                // Derive size of arguments.
                // Most functions have a fixed-size argument block,
                // so we can use metadata about the function f.
                // Not all, though: there are some variadic functions
                // in package runtime and reflect, and for those we use call-specific
                // metadata recorded by f's caller.
                if callback != nil || printing {
                        frame.argp = frame.fp
                        if usesLR {
                                frame.argp += ptrSize
                        }
                        setArgInfo(&frame, f, callback != nil)
                }

                // Determine frame's 'continuation PC', where it can continue.
                // Normally this is the return address on the stack, but if sigpanic
                // is immediately below this function on the stack, then the frame
                // stopped executing due to a trap, and frame.pc is probably not
                // a safe point for looking up liveness information. In this panicking case,
                // the function either doesn't return at all (if it has no defers or if the
                // defers do not recover) or it returns from one of the calls to
                // deferproc a second time (if the corresponding deferred func recovers).
                // It suffices to assume that the most recent deferproc is the one that
                // returns; everything live at earlier deferprocs is still live at that one.
                frame.continpc = frame.pc
                if waspanic {
                        if _defer != nil && _defer.sp == frame.sp {
                                frame.continpc = _defer.pc
                        } else {
                                frame.continpc = 0
                        }
                }

                // Unwind our local defer stack past this frame.
                for _defer != nil && (_defer.sp == frame.sp || _defer.sp == _NoArgs) {
                        _defer = _defer.link
                }

                if skip > 0 {
                        skip--
                        goto skipped
                }

                if pcbuf != nil {
                        (*[1 << 20]uintptr)(unsafe.Pointer(pcbuf))[n] = frame.pc
                }
                if callback != nil {
                        if !callback((*stkframe)(noescape(unsafe.Pointer(&frame))), v) {
                                return n
                        }
                }
                if printing {
                        if (flags&_TraceRuntimeFrames) != 0 || showframe(f, gp) {
                                // Print during crash.
                                //      main(0x1, 0x2, 0x3)
                                //              /home/rsc/go/src/runtime/x.go:23 +0xf
                                //
                                tracepc := frame.pc // back up to CALL instruction for funcline.
                                if (n > 0 || flags&_TraceTrap == 0) && frame.pc > f.entry && !waspanic {
                                        tracepc--
                                }
                                print(funcname(f), "(")
                                argp := (*[100]uintptr)(unsafe.Pointer(frame.argp))
                                for i := uintptr(0); i < frame.arglen/ptrSize; i++ {
                                        if i >= 10 {
                                                print(", ...")
                                                break
                                        }
                                        if i != 0 {
                                                print(", ")
                                        }
                                        print(hex(argp[i]))
                                }
                                print(")\n")
                                file, line := funcline(f, tracepc)
                                print("\t", file, ":", line)
                                if frame.pc > f.entry {
                                        print(" +", hex(frame.pc-f.entry))
                                }
                                if g.m.throwing > 0 && gp == g.m.curg || gotraceback >= 2 {
                                        print(" fp=", hex(frame.fp), " sp=", hex(frame.sp))
                                }
                                print("\n")
                                nprint++
                        }
                }
                n++

        skipped:
                waspanic = f.entry == sigpanicPC

                // Do not unwind past the bottom of the stack.
                if flr == nil {
                        break
                }

                // Unwind to next frame.
                frame.fn = flr
                frame.pc = frame.lr
                frame.lr = 0
                frame.sp = frame.fp
                frame.fp = 0
                frame.argmap = nil

                // On link register architectures, sighandler saves the LR on stack
                // before faking a call to sigpanic.
                if usesLR && waspanic {
                        x := *(*uintptr)(unsafe.Pointer(frame.sp))
                        frame.sp += ptrSize
                        if GOARCH == "arm64" {
                                // arm64 needs 16-byte aligned SP, always
                                frame.sp += ptrSize
                        }
                        f = findfunc(frame.pc)
                        frame.fn = f
                        if f == nil {
                                frame.pc = x
                        } else if funcspdelta(f, frame.pc) == 0 {
                                frame.lr = x
                        }
                }
        }

        if printing {
                n = nprint
        }

        // If callback != nil, we're being called to gather stack information during
        // garbage collection or stack growth. In that context, require that we used
        // up the entire defer stack. If not, then there is a bug somewhere and the
        // garbage collection or stack growth may not have seen the correct picture
        // of the stack. Crash now instead of silently executing the garbage collection
        // or stack copy incorrectly and setting up for a mysterious crash later.
        //
        // Note that panic != nil is okay here: there can be leftover panics,
        // because the defers on the panic stack do not nest in frame order as
        // they do on the defer stack. If you have:
        //
        //      frame 1 defers d1
        //      frame 2 defers d2
        //      frame 3 defers d3
        //      frame 4 panics
        //      frame 4's panic starts running defers
        //      frame 5, running d3, defers d4
        //      frame 5 panics
        //      frame 5's panic starts running defers
        //      frame 6, running d4, garbage collects
        //      frame 6, running d2, garbage collects
        //
        // During the execution of d4, the panic stack is d4 -> d3, which
        // is nested properly, and we'll treat frame 3 as resumable, because we
        // can find d3. (And in fact frame 3 is resumable. If d4 recovers
        // and frame 5 continues running, d3, d3 can recover and we'll
        // resume execution in (returning from) frame 3.)
        //
        // During the execution of d2, however, the panic stack is d2 -> d3,
        // which is inverted. The scan will match d2 to frame 2 but having
        // d2 on the stack until then means it will not match d3 to frame 3.
        // This is okay: if we're running d2, then all the defers after d2 have
        // completed and their corresponding frames are dead. Not finding d3
        // for frame 3 means we'll set frame 3's continpc == 0, which is correct
        // (frame 3 is dead). At the end of the walk the panic stack can thus
        // contain defers (d3 in this case) for dead frames. The inversion here
        // always indicates a dead frame, and the effect of the inversion on the
        // scan is to hide those dead frames, so the scan is still okay:
        // what's left on the panic stack are exactly (and only) the dead frames.
        //
        // We require callback != nil here because only when callback != nil
        // do we know that gentraceback is being called in a "must be correct"
        // context as opposed to a "best effort" context. The tracebacks with
        // callbacks only happen when everything is stopped nicely.
        // At other times, such as when gathering a stack for a profiling signal
        // or when printing a traceback during a crash, everything may not be
        // stopped nicely, and the stack walk may not be able to complete.
        // It's okay in those situations not to use up the entire defer stack:
        // incomplete information then is still better than nothing.
        if callback != nil && n < max && _defer != nil {
                if _defer != nil {
                        print("runtime: g", gp.goid, ": leftover defer sp=", hex(_defer.sp), " pc=", hex(_defer.pc), "\n")
                }
                for _defer = gp._defer; _defer != nil; _defer = _defer.link {
                        print("\tdefer ", _defer, " sp=", hex(_defer.sp), " pc=", hex(_defer.pc), "\n")
                }
                throw("traceback has leftover defers")
        }

        if callback != nil && n < max && len(stkbar) > 0 {
                print("runtime: g", gp.goid, ": leftover stack barriers ")
                gcPrintStkbars(stkbar)
                print("\n")
                throw("traceback has leftover stack barriers")
        }

        if callback != nil && n < max && frame.sp != gp.stktopsp {
                print("runtime: g", gp.goid, ": frame.sp=", hex(frame.sp), " top=", hex(gp.stktopsp), "\n")
                print("\tstack=[", hex(gp.stack.lo), "-", hex(gp.stack.hi), "] n=", n, " max=", max, "\n")
                throw("traceback did not unwind completely")
        }

        return n
}

func setArgInfo(frame *stkframe, f *_func, needArgMap bool) {
        frame.arglen = uintptr(f.args)
        if needArgMap && f.args == _ArgsSizeUnknown {
                // Extract argument bitmaps for reflect stubs from the calls they made to reflect.
                switch funcname(f) {
                case "reflect.makeFuncStub", "reflect.methodValueCall":
                        arg0 := frame.sp
                        if usesLR {
                                arg0 += ptrSize
                        }
                        fn := *(**[2]uintptr)(unsafe.Pointer(arg0))
                        if fn[0] != f.entry {
                                print("runtime: confused by ", funcname(f), "\n")
                                throw("reflect mismatch")
                        }
                        bv := (*bitvector)(unsafe.Pointer(fn[1]))
                        frame.arglen = uintptr(bv.n * ptrSize)
                        frame.argmap = bv
                }
        }
}

func printcreatedby(gp *g) {
        // Show what created goroutine, except main goroutine (goid 1).
        pc := gp.gopc
        f := findfunc(pc)
        if f != nil && showframe(f, gp) && gp.goid != 1 {
                print("created by ", funcname(f), "\n")
                tracepc := pc // back up to CALL instruction for funcline.
                if pc > f.entry {
                        tracepc -= _PCQuantum
                }
                file, line := funcline(f, tracepc)
                print("\t", file, ":", line)
                if pc > f.entry {
                        print(" +", hex(pc-f.entry))
                }
                print("\n")
        }
}

func traceback(pc, sp, lr uintptr, gp *g) {
        traceback1(pc, sp, lr, gp, 0)
}

// tracebacktrap is like traceback but expects that the PC and SP were obtained
// from a trap, not from gp->sched or gp->syscallpc/gp->syscallsp or getcallerpc/getcallersp.
// Because they are from a trap instead of from a saved pair,
// the initial PC must not be rewound to the previous instruction.
// (All the saved pairs record a PC that is a return address, so we
// rewind it into the CALL instruction.)
func tracebacktrap(pc, sp, lr uintptr, gp *g) {
        traceback1(pc, sp, lr, gp, _TraceTrap)
}

func traceback1(pc, sp, lr uintptr, gp *g, flags uint) {
        var n int
        if readgstatus(gp)&^_Gscan == _Gsyscall {
                // Override registers if blocked in system call.
                pc = gp.syscallpc
                sp = gp.syscallsp
                flags &^= _TraceTrap
        }
        // Print traceback. By default, omits runtime frames.
        // If that means we print nothing at all, repeat forcing all frames printed.
        n = gentraceback(pc, sp, lr, gp, 0, nil, _TracebackMaxFrames, nil, nil, flags)
        if n == 0 && (flags&_TraceRuntimeFrames) == 0 {
                n = gentraceback(pc, sp, lr, gp, 0, nil, _TracebackMaxFrames, nil, nil, flags|_TraceRuntimeFrames)
        }
        if n == _TracebackMaxFrames {
                print("...additional frames elided...\n")
        }
        printcreatedby(gp)
}

func callers(skip int, pcbuf []uintptr) int {
        sp := getcallersp(unsafe.Pointer(&skip))
        pc := uintptr(getcallerpc(unsafe.Pointer(&skip)))
        gp := getg()
        var n int
        systemstack(func() {
                n = gentraceback(pc, sp, 0, gp, skip, &pcbuf[0], len(pcbuf), nil, nil, 0)
        })
        return n
}

func gcallers(gp *g, skip int, pcbuf []uintptr) int {
        return gentraceback(^uintptr(0), ^uintptr(0), 0, gp, skip, &pcbuf[0], len(pcbuf), nil, nil, 0)
}

func showframe(f *_func, gp *g) bool {
        g := getg()
        if g.m.throwing > 0 && gp != nil && (gp == g.m.curg || gp == g.m.caughtsig.ptr()) {
                return true
        }
        traceback := gotraceback(nil)
        name := funcname(f)

        // Special case: always show runtime.panic frame, so that we can
        // see where a panic started in the middle of a stack trace.
        // See golang.org/issue/5832.
        if name == "runtime.panic" {
                return true
        }

        return traceback > 1 || f != nil && contains(name, ".") && (!hasprefix(name, "runtime.") || isExportedRuntime(name))
}

// isExportedRuntime reports whether name is an exported runtime function.
// It is only for runtime functions, so ASCII A-Z is fine.
func isExportedRuntime(name string) bool {
        const n = len("runtime.")
        return len(name) > n && name[:n] == "runtime." && 'A' <= name[n] && name[n] <= 'Z'
}

var gStatusStrings = [...]string{
        _Gidle:      "idle",
        _Grunnable:  "runnable",
        _Grunning:   "running",
        _Gsyscall:   "syscall",
        _Gwaiting:   "waiting",
        _Gdead:      "dead",
        _Genqueue:   "enqueue",
        _Gcopystack: "copystack",
}

var gScanStatusStrings = [...]string{
        0:          "scan",
        _Grunnable: "scanrunnable",
        _Grunning:  "scanrunning",
        _Gsyscall:  "scansyscall",
        _Gwaiting:  "scanwaiting",
        _Gdead:     "scandead",
        _Genqueue:  "scanenqueue",
}

func goroutineheader(gp *g) {
        gpstatus := readgstatus(gp)

        // Basic string status
        var status string
        if 0 <= gpstatus && gpstatus < uint32(len(gStatusStrings)) {
                status = gStatusStrings[gpstatus]
        } else if gpstatus&_Gscan != 0 && 0 <= gpstatus&^_Gscan && gpstatus&^_Gscan < uint32(len(gStatusStrings)) {
                status = gStatusStrings[gpstatus&^_Gscan]
        } else {
                status = "???"
        }

        // Override.
        if (gpstatus == _Gwaiting || gpstatus == _Gscanwaiting) && gp.waitreason != "" {
                status = gp.waitreason
        }

        // approx time the G is blocked, in minutes
        var waitfor int64
        gpstatus &^= _Gscan // drop the scan bit
        if (gpstatus == _Gwaiting || gpstatus == _Gsyscall) && gp.waitsince != 0 {
                waitfor = (nanotime() - gp.waitsince) / 60e9
        }
        print("goroutine ", gp.goid, " [", status)
        if waitfor >= 1 {
                print(", ", waitfor, " minutes")
        }
        if gp.lockedm != nil {
                print(", locked to thread")
        }
        print("]:\n")
}

func tracebackothers(me *g) {
        level := gotraceback(nil)

        // Show the current goroutine first, if we haven't already.
        g := getg()
        gp := g.m.curg
        if gp != nil && gp != me {
                print("\n")
                goroutineheader(gp)
                traceback(^uintptr(0), ^uintptr(0), 0, gp)
        }

        lock(&allglock)
        for _, gp := range allgs {
                if gp == me || gp == g.m.curg || readgstatus(gp) == _Gdead || isSystemGoroutine(gp) && level < 2 {
                        continue
                }
                print("\n")
                goroutineheader(gp)
                // Note: gp.m == g.m occurs when tracebackothers is
                // called from a signal handler initiated during a
                // systemstack call.  The original G is still in the
                // running state, and we want to print its stack.
                if gp.m != g.m && readgstatus(gp)&^_Gscan == _Grunning {
                        print("\tgoroutine running on other thread; stack unavailable\n")
                        printcreatedby(gp)
                } else {
                        traceback(^uintptr(0), ^uintptr(0), 0, gp)
                }
        }
        unlock(&allglock)
}

// Does f mark the top of a goroutine stack?
func topofstack(f *_func) bool {
        pc := f.entry
        return pc == goexitPC ||
                pc == mstartPC ||
                pc == mcallPC ||
                pc == morestackPC ||
                pc == rt0_goPC ||
                externalthreadhandlerp != 0 && pc == externalthreadhandlerp
}

// isSystemGoroutine reports whether the goroutine g must be omitted in
// stack dumps and deadlock detector.
func isSystemGoroutine(gp *g) bool {
        pc := gp.startpc
        return pc == runfinqPC && !fingRunning ||
                pc == backgroundgcPC ||
                pc == bgsweepPC ||
                pc == forcegchelperPC ||
                pc == timerprocPC ||
                pc == gcBgMarkWorkerPC
}