goroutine 退出过程// The top-most function running on a goroutin

// The top-most function running on a goroutine
// returns to goexit+PCQuantum.
TEXT runtime·goexit(SB),NOSPLIT|TOPFRAME,$0-0
   BYTE   $0x90  // NOP
   CALL   runtime·goexit1(SB)    // does not return
   // traceback from goexit1 must hit code range of goexit
   BYTE   $0x90  // NOP

goexit1

// Finishes execution of the current goroutine.
func goexit1() {
   if raceenabled {
      racegoend()
   }
   if trace.enabled {
      traceGoEnd()
   }
   mcall(goexit0)//g0 栈上执行

}

goexit0

-->回收g,放入_p_.gFree -->schedule()