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// Called from runtime·morestack when more stack is needed.
// Allocate larger stack and relocate to new stack.
// Stack growth is multiplicative, for constant amortized cost.
//
// g->atomicstatus will be Grunning or Gscanrunning upon entry.
// If the scheduler is trying to stop this g, then it will set preemptStop.
//
// This must be nowritebarrierrec because it can be called as part of
// stack growth from other nowritebarrierrec functions, but the
// compiler doesn't check this.
//
//go:nowritebarrierrec
func newstack() {
thisg := getg() // thisg = g0; 根据morestack()函数的相关代码
// TODO: double check all gp. shouldn't be getg().
//
// 根据morestack()函数的相关代码，这里thisg.m.morebuf.g.ptr()是g不是g0
if thisg.m.morebuf.g.ptr().stackguard0 == stackFork {
throw("stack growth after fork")
}
// m->curg 是当前m上正在运行的g
if thisg.m.morebuf.g.ptr() != thisg.m.curg {
print("runtime: newstack called from g=", hex(thisg.m.morebuf.g), "\n"+"\tm=", thisg.m, " m->curg=", thisg.m.curg, " m->g0=", thisg.m.g0, " m->gsignal=", thisg.m.gsignal, "\n")
morebuf := thisg.m.morebuf
traceback(morebuf.pc, morebuf.sp, morebuf.lr, morebuf.g.ptr())
throw("runtime: wrong goroutine in newstack")
}
gp := thisg.m.curg // gp 在这里例子是runtime.main的goroutine
// g.throwsplit 在系统调用前会被设置为true或其他地方。因此g出现在这里不合适。
if thisg.m.curg.throwsplit {
// Update syscallsp, syscallpc in case traceback uses them.
morebuf := thisg.m.morebuf
gp.syscallsp = morebuf.sp
gp.syscallpc = morebuf.pc
pcname, pcoff := "(unknown)", uintptr(0)
f := findfunc(gp.sched.pc)
if f.valid() {
pcname = funcname(f)
pcoff = gp.sched.pc - f.entry()
}
print("runtime: newstack at ", pcname, "+", hex(pcoff),
" sp=", hex(gp.sched.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")
thisg.m.traceback = 2 // Include runtime frames
traceback(morebuf.pc, morebuf.sp, morebuf.lr, gp)
throw("runtime: stack split at bad time")
}
// m.morebuf 在上面的morestack函数中被设置为调用函数的相关信息。
morebuf := thisg.m.morebuf
thisg.m.morebuf.pc = 0
thisg.m.morebuf.lr = 0
thisg.m.morebuf.sp = 0
thisg.m.morebuf.g = 0
// NOTE: stackguard0 may change underfoot, if another thread
// is about to try to preempt gp. Read it just once and use that same
// value now and below.
//
// 注意：如果另一个线程即将尝试抢占gp，stackguard0可能会在脚下发生变化。
// 只需阅读一次并在现在和下面使用相同的值
stackguard0 := atomic.Loaduintptr(&gp.stackguard0) // 获取gp.stackguard0
// 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.
preempt := stackguard0 == stackPreempt // 判断当前是否真需要被抢占
if preempt {
// canPreemptM -> mp.locks == 0 && mp.mallocing == 0 && mp.preemptoff == "" && mp.p.ptr().status == _Prunning
if !canPreemptM(thisg.m) { // canPreemptM(thisg.m); true.可以抢占; false.不允许抢占
// 以下是【不允许】抢占时，再次恢复gp。
// Let the goroutine keep running for now.
// gp->preempt is set, so it will be preempted next time.
//
// 现在让goroutine继续运行。gp->preempt已设置，因此下次将被抢占。
// (gp->preempt在前面已被设置为true)
// 还原stackguard0为正常值，表示我们已经处理过抢占请求了
gp.stackguard0 = gp.stack.lo + _StackGuard
// 恢复gp，这里永远不会返回
gogo(&gp.sched) // never return
}
}
if gp.stack.lo == 0 {
throw("missing stack in newstack")
}
sp := gp.sched.sp
if goarch.ArchFamily == goarch.AMD64 || goarch.ArchFamily == goarch.I386 || goarch.ArchFamily == goarch.WASM {
// The call to morestack cost a word.
sp -= goarch.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, ", goid=", gp.goid, ", gp->status=", hex(readgstatus(gp)), "\n ")
print("runtime: split stack overflow: ", hex(sp), " < ", hex(gp.stack.lo), "\n")
throw("runtime: split stack overflow")
}
// 判断抢占，发起抢占
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")
}
if gp.preemptShrink {
// We're at a synchronous safe point now, so
// do the pending stack shrink.
gp.preemptShrink = false
shrinkstack(gp)
}
// 停止抢占，开启下一次调度循环,makeroot期间改值会被设置为true。
if gp.preemptStop {
preemptPark(gp) // never returns
}
// Act like goroutine called runtime.Gosched.
//
// 像调用 runtime.Gosched 的 goroutine 一样
// 调用gopreempt_m把gp切换出去，抢占这个goroutine成功了
gopreempt_m(gp) // never return
}
// 下面代码是扩大栈相关代码
// Allocate a bigger segment and move the stack.
oldsize := gp.stack.hi - gp.stack.lo
newsize := oldsize * 2 // 扩大为原来的2倍
// Make sure we grow at least as much as needed to fit the new frame.
// (This is just an optimization - the caller of morestack will
// recheck the bounds on return.)
if f := findfunc(gp.sched.pc); f.valid() {
max := uintptr(funcMaxSPDelta(f))
needed := max + _StackGuard
used := gp.stack.hi - gp.sched.sp
for newsize-used < needed {
newsize *= 2
}
}
if stackguard0 == stackForceMove {
// Forced stack movement used for debugging.
// Don't double the stack (or we may quickly run out
// if this is done repeatedly).
newsize = oldsize
}
if newsize > maxstacksize || newsize > maxstackceiling {
if maxstacksize < maxstackceiling {
print("runtime: goroutine stack exceeds ", maxstacksize, "-byte limit\n")
} else {
print("runtime: goroutine stack exceeds ", maxstackceiling, "-byte limit\n")
}
print("runtime: sp=", hex(sp), " stack=[", hex(gp.stack.lo), ", ", hex(gp.stack.hi), "]\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, newsize)
if stackDebug >= 1 {
print("stack grow done\n")
}
casgstatus(gp, _Gcopystack, _Grunning)
gogo(&gp.sched) // 再次恢复这个goroutine
}
|
