go语言调度gmp原理(5)

线程管理

go语言的运行时会通过调度器改变线程的所有权，它也提供了runtime.lockOSthread和runtime.UnlockOSthread,让我们能绑定goroutine和线程完成一些比较特殊的操作。goroutine应该在调用操作系统服务或者依赖线程状态的非go语言库时调用runtime.lockOSThread函数，例如C语言图形库等

runtime.lockOSThread会通过如下所示的代码绑定goroutine和当前线程

func lockOSThread() {
	getg().m.lockedInt++
	dolockOSThread()
}

func dolockOSThread() {
	if GOARCH == "wasm" {
		return // no threads on wasm yet
	}
	gp := getg()
	gp.m.lockedg.set(gp)
	gp.lockedm.set(gp.m)
}

runtime.dolockOSThread会分别设置线程的lockedg字段和goroutine的lockedm字段，这两行代码会绑定线程和goroutine

当goroutine完成特定操作之后，会调用函数runtime.UnlockOSThread分离goroutine和线程

func unlockOSThread() {
	gp := getg()
	if gp.m.lockedInt == 0 {
		systemstack(badunlockosthread)
	}
	gp.m.lockedInt--
	dounlockOSThread()
}

func dounlockOSThread() {
	if GOARCH == "wasm" {
		return // no threads on wasm yet
	}
	gp := getg()
	if gp.m.lockedInt != 0 || gp.m.lockedExt != 0 {
		return
	}
	gp.m.lockedg = 0
	gp.lockedm = 0
}

函数执行的过程与runtime.LockOSThread正好相反。在多数服务之中，我们用不到这对函数

线程的生命周期

go语言的运行时会通过runtime.startm启动线程来执行处理器P，如果我们在该函数中没能从空闲列表中获取线程M，就会调用runtime.newm创建新线程

func newm(fn func(), pp *p, id int64) {
	// allocm adds a new M to allm, but they do not start until created by
	// the OS in newm1 or the template thread.
	//
	// doAllThreadsSyscall requires that every M in allm will eventually
	// start and be signal-able, even with a STW.
	//
	// Disable preemption here until we start the thread to ensure that
	// newm is not preempted between allocm and starting the new thread,
	// ensuring that anything added to allm is guaranteed to eventually
	// start.
	acquirem()

	mp := allocm(pp, fn, id)
	mp.nextp.set(pp)
	mp.sigmask = initSigmask
	if gp := getg(); gp != nil && gp.m != nil && (gp.m.lockedExt != 0 || gp.m.incgo) && GOOS != "plan9" {
		// We're on a locked M or a thread that may have been
		// started by C. The kernel state of this thread may
		// be strange (the user may have locked it for that
		// purpose). We don't want to clone that into another
		// thread. Instead, ask a known-good thread to create
		// the thread for us.
		//
		// This is disabled on Plan 9. See golang.org/issue/22227.
		//
		// TODO: This may be unnecessary on Windows, which
		// doesn't model thread creation off fork.
		lock(&newmHandoff.lock)
		if newmHandoff.haveTemplateThread == 0 {
			throw("on a locked thread with no template thread")
		}
		mp.schedlink = newmHandoff.newm
		newmHandoff.newm.set(mp)
		if newmHandoff.waiting {
			newmHandoff.waiting = false
			notewakeup(&newmHandoff.wake)
		}
		unlock(&newmHandoff.lock)
		// The M has not started yet, but the template thread does not
		// participate in STW, so it will always process queued Ms and
		// it is safe to releasem.
		releasem(getg().m)
		return
	}
	newm1(mp)
	releasem(getg().m)
}

func newm1(mp *m) {
	if iscgo {
		var ts cgothreadstart
		if _cgo_thread_start == nil {
			throw("_cgo_thread_start missing")
		}
		ts.g.set(mp.g0)
		ts.tls = (*uint64)(unsafe.Pointer(&mp.tls[0]))
		ts.fn = unsafe.Pointer(abi.FuncPCABI0(mstart))
		if msanenabled {
			msanwrite(unsafe.Pointer(&ts), unsafe.Sizeof(ts))
		}
		if asanenabled {
			asanwrite(unsafe.Pointer(&ts), unsafe.Sizeof(ts))
		}
		execLock.rlock() // Prevent process clone.
		asmcgocall(_cgo_thread_start, unsafe.Pointer(&ts))
		execLock.runlock()
		return
	}
	execLock.rlock() // Prevent process clone.
	newosproc(mp)
	execLock.runlock()
}

创建新线程需要使用如下所示的runtime.newosproc，该函数在Linux平台上会通过系统调用clone创建新的操作系统线程，它也是创建线程链路上距离操作系统最近的go语言函数

func newosproc(mp *m) {
	// We pass 0 for the stack size to use the default for this binary.
	thandle := stdcall6(_CreateThread, 0, 0,
		abi.FuncPCABI0(tstart_stdcall), uintptr(unsafe.Pointer(mp)),
		0, 0)

	if thandle == 0 {
		if atomic.Load(&exiting) != 0 {
			// CreateThread may fail if called
			// concurrently with ExitProcess. If this
			// happens, just freeze this thread and let
			// the process exit. See issue #18253.
			lock(&deadlock)
			lock(&deadlock)
		}
		print("runtime: failed to create new OS thread (have ", mcount(), " already; errno=", getlasterror(), ")\n")
		throw("runtime.newosproc")
	}

	// Close thandle to avoid leaking the thread object if it exits.
	stdcall1(_CloseHandle, thandle)
}

使用系统调用clone创建的线程会在线程主动调用exit或者传入的函数runtime.mstart返回时主动退出，runtime.mstart会执行调用runtime.newm时传入的匿名函数fn，至此也就完成了从线程创建到销毁的整个闭环