stack.go

	preempt := atomic.Loaduintptr(&gp.stackguard0) == stackPreempt

	// Be conservative about where we preempt.
	// We are interested in preempting user Go code, not runtime code.
	// If we're holding locks, mallocing, or preemption is disabled, don't
	// preempt.
	// This check is very early in newstack so that even the status change
	// from Grunning to Gwaiting and back doesn't happen in this case.
	// That status change by itself can be viewed as a small preemption,
	// because the GC might change Gwaiting to Gscanwaiting, and then
	// this goroutine has to wait for the GC to finish before continuing.
	// If the GC is in some way dependent on this goroutine (for example,
	// it needs a lock held by the goroutine), that small preemption turns
	// into a real deadlock.
	if preempt {
		if thisg.m.locks != 0 || thisg.m.mallocing != 0 || thisg.m.preemptoff != "" || thisg.m.p.ptr().status != _Prunning {
			// Let the goroutine keep running for now.
			// gp->preempt is set, so it will be preempted next time.
			gp.stackguard0 = gp.stack.lo + _StackGuard
			gogo(&gp.sched) // never return
		}
	}

	if gp.stack.lo == 0 {
		throw("missing stack in newstack")
	}
	sp := gp.sched.sp
	if sys.ArchFamily == sys.AMD64 || sys.ArchFamily == sys.I386 {
		// The call to morestack cost a word.
		sp -= sys.PtrSize
	}
	if stackDebug >= 1 || sp < gp.stack.lo {
		print("runtime: newstack sp=", hex(sp), " stack=[", hex(gp.stack.lo), ", ", hex(gp.stack.hi), "]\n",
			"\tmorebuf={pc:", hex(morebuf.pc), " sp:", hex(morebuf.sp), " lr:", hex(morebuf.lr), "}\n",
			"\tsched={pc:", hex(gp.sched.pc), " sp:", hex(gp.sched.sp), " lr:", hex(gp.sched.lr), " ctxt:", gp.sched.ctxt, "}\n")
	}
	if sp < gp.stack.lo {
		print("runtime: gp=", gp, ", gp->status=", hex(readgstatus(gp)), "\n ")
		print("runtime: split stack overflow: ", hex(sp), " < ", hex(gp.stack.lo), "\n")
		throw("runtime: split stack overflow")
	}

	if preempt {
		if gp == thisg.m.g0 {
			throw("runtime: preempt g0")
		}
		if thisg.m.p == 0 && thisg.m.locks == 0 {
			throw("runtime: g is running but p is not")
		}
		// Synchronize with scang.
		casgstatus(gp, _Grunning, _Gwaiting)
		if gp.preemptscan {
			for !castogscanstatus(gp, _Gwaiting, _Gscanwaiting) {
				// Likely to be racing with the GC as
				// it sees a _Gwaiting and does the
				// stack scan. If so, gcworkdone will
				// be set and gcphasework will simply
				// return.
			}
			if !gp.gcscandone {
				// gcw is safe because we're on the
				// system stack.
				gcw := &gp.m.p.ptr().gcw
				scanstack(gp, gcw)
				if gcBlackenPromptly {
					gcw.dispose()
				}
				gp.gcscandone = true
			}
			gp.preemptscan = false
			gp.preempt = false
			casfrom_Gscanstatus(gp, _Gscanwaiting, _Gwaiting)
			// This clears gcscanvalid.
			casgstatus(gp, _Gwaiting, _Grunning)
			gp.stackguard0 = gp.stack.lo + _StackGuard
			gogo(&gp.sched) // never return
		}

		// Act like goroutine called runtime.Gosched.
		casgstatus(gp, _Gwaiting, _Grunning)
		gopreempt_m(gp) // never return
	}

	// Allocate a bigger segment and move the stack.
	oldsize := int(gp.stackAlloc)
	newsize := oldsize * 2
	if uintptr(newsize) > maxstacksize {
		print("runtime: goroutine stack exceeds ", maxstacksize, "-byte limit\n")
		throw("stack overflow")
	}

	// The goroutine must be executing in order to call newstack,
	// so it must be Grunning (or Gscanrunning).
	casgstatus(gp, _Grunning, _Gcopystack)

	// The concurrent GC will not scan the stack while we are doing the copy since
	// the gp is in a Gcopystack status.
	copystack(gp, uintptr(newsize), true)
	if stackDebug >= 1 {
		print("stack grow done\n")
	}
	casgstatus(gp, _Gcopystack, _Grunning)
	gogo(&gp.sched)
}

//go:nosplit
func nilfunc() {
	*(*uint8)(nil) = 0
}

// adjust Gobuf as if it executed a call to fn
// and then did an immediate gosave.
func gostartcallfn(gobuf *gobuf, fv *funcval) {
	var fn unsafe.Pointer
	if fv != nil {
		fn = unsafe.Pointer(fv.fn)
	} else {
		fn = unsafe.Pointer(funcPC(nilfunc))
	}
	gostartcall(gobuf, fn, unsafe.Pointer(fv))
}

// Maybe shrink the stack being used by gp.
// Called at garbage collection time.
// gp must be stopped, but the world need not be.
func shrinkstack(gp *g) {
	gstatus := readgstatus(gp)
	if gstatus&^_Gscan == _Gdead {
		if gp.stack.lo != 0 {
			// Free whole stack - it will get reallocated
			// if G is used again.
			stackfree(gp.stack, gp.stackAlloc)
			gp.stack.lo = 0
			gp.stack.hi = 0
			gp.stkbar = nil
			gp.stkbarPos = 0
		}
		return
	}
	if gp.stack.lo == 0 {
		throw("missing stack in shrinkstack")
	}
	if gstatus&_Gscan == 0 {
		throw("bad status in shrinkstack")
	}

	if debug.gcshrinkstackoff > 0 {
		return
	}
	if gp.startpc == gcBgMarkWorkerPC {
		// We're not allowed to shrink the gcBgMarkWorker
		// stack (see gcBgMarkWorker for explanation).
		return
	}

	oldsize := gp.stackAlloc
	newsize := oldsize / 2
	// Don't shrink the allocation below the minimum-sized stack
	// allocation.
	if newsize < _FixedStack {
		return
	}
	// Compute how much of the stack is currently in use and only
	// shrink the stack if gp is using less than a quarter of its
	// current stack. The currently used stack includes everything
	// down to the SP plus the stack guard space that ensures
	// there's room for nosplit functions.
	avail := gp.stack.hi - gp.stack.lo
	if used := gp.stack.hi - gp.sched.sp + _StackLimit; used >= avail/4 {
		return
	}

	// We can't copy the stack if we're in a syscall.
	// The syscall might have pointers into the stack.
	if gp.syscallsp != 0 {
		return
	}
	if sys.GoosWindows != 0 && gp.m != nil && gp.m.libcallsp != 0 {
		return
	}

	if stackDebug > 0 {
		print("shrinking stack ", oldsize, "->", newsize, "\n")
	}

	copystack(gp, newsize, false)
}

// freeStackSpans frees unused stack spans at the end of GC.
func freeStackSpans() {
	lock(&stackpoolmu)

	// Scan stack pools for empty stack spans.
	for order := range stackpool {
		list := &stackpool[order]
		for s := list.first; s != nil; {
			next := s.next
			if s.allocCount == 0 {
				list.remove(s)
				s.stackfreelist = 0
				mheap_.freeStack(s)
			}
			s = next
		}
	}

	unlock(&stackpoolmu)

	// Free large stack spans.
	lock(&stackLarge.lock)
	for i := range stackLarge.free {
		for s := stackLarge.free[i].first; s != nil; {
			next := s.next
			stackLarge.free[i].remove(s)
			mheap_.freeStack(s)
			s = next
		}
	}
	unlock(&stackLarge.lock)
}

//go:nosplit
func morestackc() {
	systemstack(func() {
		throw("attempt to execute C code on Go stack")
	})
}

//go:nosplit
func inStack(p uintptr, s stack) bool {
	return s.lo <= p && p < s.hi
}