package main

import (
   "fmt"
   "io/ioutil"
   "os"
)

func main() {
   f, err := os.Open("./file")
   if err != nil {
      panic(err)
   }
   defer f.Close()

   content, err := ioutil.ReadAll(f)
   if err != nil {
      panic(err)
   }
   fmt.Println(string(content))
}

func entersyscall() {
   reentersyscall(getcallerpc(), getcallersp())
}

func reentersyscall(pc, sp uintptr) {
   _g_ := getg()

   // Disable preemption because during this function g is in Gsyscall status,
   // but can have inconsistent g->sched, do not let GC observe it.
   _g_.m.locks++

   // Entersyscall must not call any function that might split/grow the stack.
   // (See details in comment above.)
   // Catch calls that might, by replacing the stack guard with something that
   // will trip any stack check and leaving a flag to tell newstack to die.
   _g_.stackguard0 = stackPreempt
   _g_.throwsplit = true

   // Leave SP around for GC and traceback.
   save(pc, sp)  // 保存pc和sp
   _g_.syscallsp = sp
   _g_.syscallpc = pc
   casgstatus(_g_, _Grunning, _Gsyscall)
   if _g_.syscallsp < _g_.stack.lo || _g_.stack.hi < _g_.syscallsp {
      systemstack(func() {
         print("entersyscall inconsistent ", hex(_g_.syscallsp), " [", hex(_g_.stack.lo), ",", hex(_g_.stack.hi), "]\n")
         throw("entersyscall")
      })
   }

   if trace.enabled {
      systemstack(traceGoSysCall)
      // systemstack itself clobbers g.sched.{pc,sp} and we might
      // need them later when the G is genuinely blocked in a
      // syscall
      save(pc, sp)
   }

   if atomic.Load(&sched.sysmonwait) != 0 {
      systemstack(entersyscall_sysmon)
      save(pc, sp)
   }

   if _g_.m.p.ptr().runSafePointFn != 0 {
      // runSafePointFn may stack split if run on this stack
      systemstack(runSafePointFn)
      save(pc, sp)
   }

   // 一下解綁P和M
   _g_.m.syscalltick = _g_.m.p.ptr().syscalltick
   _g_.sysblocktraced = true
   pp := _g_.m.p.ptr()
   pp.m = 0
   _g_.m.oldp.set(pp)  // 存儲一下舊P
   _g_.m.p = 0
   atomic.Store(&pp.status, _Psyscall)
   if sched.gcwaiting != 0 {
      systemstack(entersyscall_gcwait)
      save(pc, sp)
   }

   _g_.m.locks--
}

func exitsyscall() {
   _g_ := getg()

   _g_.m.locks++ // see comment in entersyscall
   if getcallersp() > _g_.syscallsp {
      throw("exitsyscall: syscall frame is no longer valid")
   }

   _g_.waitsince = 0
   oldp := _g_.m.oldp.ptr()  // 拿到開始存儲的舊P
   _g_.m.oldp = 0
   if exitsyscallfast(oldp) {
      if trace.enabled {
         if oldp != _g_.m.p.ptr() || _g_.m.syscalltick != _g_.m.p.ptr().syscalltick {
            systemstack(traceGoStart)
         }
      }
      // There's a cpu for us, so we can run.
      _g_.m.p.ptr().syscalltick++
      // We need to cas the status and scan before resuming...
      casgstatus(_g_, _Gsyscall, _Grunning)

      ...

      return
   }

   ...

   // Call the scheduler.
   mcall(exitsyscall0)

   // Scheduler returned, so we're allowed to run now.
   // Delete the syscallsp information that we left for
   // the garbage collector during the system call.
   // Must wait until now because until gosched returns
   // we don't know for sure that the garbage collector
   // is not running.
   _g_.syscallsp = 0
   _g_.m.p.ptr().syscalltick++
   _g_.throwsplit = false
}

//go:nosplit
func exitsyscallfast(oldp *p) bool {
   _g_ := getg()

   // Freezetheworld sets stopwait but does not retake P's.
   if sched.stopwait == freezeStopWait {
      return false
   }

   // 如果上一個P沒有被其他M占用，還處于_Psyscall狀態(tài)，那么直接通過wirep函數(shù)獲取此P
   // Try to re-acquire the last P.
   if oldp != nil && oldp.status == _Psyscall && atomic.Cas(&oldp.status, _Psyscall, _Pidle) {
      // There's a cpu for us, so we can run.
      wirep(oldp)
      exitsyscallfast_reacquired()
      return true
   }

   // Try to get any other idle P.
   if sched.pidle != 0 {
      var ok bool
      systemstack(func() {
         ok = exitsyscallfast_pidle()
         if ok && trace.enabled {
            if oldp != nil {
               // Wait till traceGoSysBlock event is emitted.
               // This ensures consistency of the trace (the goroutine is started after it is blocked).
               for oldp.syscalltick == _g_.m.syscalltick {
                  osyield()
               }
            }
            traceGoSysExit(0)
         }
      })
      if ok {
         return true
      }
   }
   return false
}

func exitsyscall0(gp *g) {
   casgstatus(gp, _Gsyscall, _Grunnable)
   dropg() // 因為當(dāng)前m沒有找到p，所以先解開g和m
   lock(&sched.lock)
   var _p_ *p
   if schedEnabled(gp) {
      _p_ = pidleget() // 還是嘗試找一下有沒有空閑的p
   }
   var locked bool
   if _p_ == nil { // 如果還是沒有空閑p，那么把g扔到全局隊列去等待調(diào)度
      globrunqput(gp)

      // Below, we stoplockedm if gp is locked. globrunqput releases
      // ownership of gp, so we must check if gp is locked prior to
      // committing the release by unlocking sched.lock, otherwise we
      // could race with another M transitioning gp from unlocked to
      // locked.
      locked = gp.lockedm != 0
   } else if atomic.Load(&sched.sysmonwait) != 0 {
      atomic.Store(&sched.sysmonwait, 0)
      notewakeup(&sched.sysmonnote)
   }
   unlock(&sched.lock)
   if _p_ != nil { // 如果找到了空閑p，那么就去執(zhí)行，這個分支永遠不會返回
      acquirep(_p_)
      execute(gp, false) // Never returns.
   }
   if locked {
      // Wait until another thread schedules gp and so m again.
      //
      // N.B. lockedm must be this M, as this g was running on this M
      // before entersyscall.
      stoplockedm()
      execute(gp, false) // Never returns.
   }
   stopm() // 這里還是沒有找到空閑p的條件，停止這個m，因為沒有p，所以m應(yīng)該要開始找工作了
   schedule() // Never returns. // 通過schedule函數(shù)進行調(diào)度
}