[Golang]OS系统调用浅析

func Read(fd int, p []byte) (n int, err error)
func Write(fd int, p []byte) (n int, err error)

func Syscall(trap, a1, a2, a3 uintptr) (r1, r2 uintptr, err Errno)
func Syscall6(trap, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2 uintptr, err Errno)

func RawSyscall(trap, a1, a2, a3 uintptr) (r1, r2 uintptr, err Errno)
func RawSyscall6(trap, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2 uintptr, err Errno)

void runtime·entersyscall(void);
void runtime·entersyscallblock(void);
void runtime·exitsyscall(void);

void ·entersyscall(int dummy) { ... }
void ·entersyscallblock(int dummy) { ... }

#pragma textflag NOSPLIT
void
·entersyscall(int32 dummy)
{
  // Disable preemption because during this function g is in Gsyscall status,
  // but can have inconsistent g->sched, do not let GC observe it.
  m->locks++;

  // Leave SP around for GC and traceback.
  save(runtime·getcallerpc(&dummy), runtime·getcallersp(&dummy));
  g->syscallsp = g->sched.sp;
  g->syscallpc = g->sched.pc;
  g->syscallstack = g->stackbase;
  g->syscallguard = g->stackguard;
  g->status = Gsyscall;
  if(g->syscallsp < g->syscallguard-StackGuard || g->syscallstack < g->syscallsp) {
      // runtime·printf("entersyscall inconsistent %p [%p,%p]\n",
      // g->syscallsp, g->syscallguard-StackGuard, g->syscallstack);
      runtime·throw("entersyscall");
  }

if(runtime·atomicload(&runtime·sched.sysmonwait)) {  // TODO: fast atomic
  runtime·lock(&runtime·sched);
  if(runtime·atomicload(&runtime·sched.sysmonwait)) {
      runtime·atomicstore(&runtime·sched.sysmonwait, 0);
      runtime·notewakeup(&runtime·sched.sysmonnote);
  }
  runtime·unlock(&runtime·sched);
  save(runtime·getcallerpc(&dummy), runtime·getcallersp(&dummy));
}

m->mcache = nil;
m->p->m = nil;
runtime·atomicstore(&m->p->status, Psyscall);
if(runtime·sched.gcwaiting) {
  runtime·lock(&runtime·sched);
  if (runtime·sched.stopwait > 0 && runtime·cas(&m->p->status, Psyscall, Pgcstop)) {
      if(--runtime·sched.stopwait == 0)
          runtime·notewakeup(&runtime·sched.stopnote);
  }
  runtime·unlock(&runtime·sched);
  save(runtime·getcallerpc(&dummy), runtime·getcallersp(&dummy));
}

// Goroutines must not split stacks in Gsyscall status (it would corrupt g->sched).
// We set stackguard to StackPreempt so that first split stack check calls morestack.
// Morestack detects this case and throws.
g->stackguard0 = StackPreempt;
m->locks--;
}

#pragma textflag NOSPLIT
void
·entersyscallblock(int32 dummy)
{
  P *p;

  m->locks++;  // see comment in entersyscall

  // Leave SP around for GC and traceback.
  save(runtime·getcallerpc(&dummy), runtime·getcallersp(&dummy));
  g->syscallsp = g->sched.sp;
  g->syscallpc = g->sched.pc;
  g->syscallstack = g->stackbase;
  g->syscallguard = g->stackguard;
  g->status = Gsyscall;
  if(g->syscallsp < g->syscallguard-StackGuard || g->syscallstack < g->syscallsp) {
      // runtime·printf("entersyscall inconsistent %p [%p,%p]\n",
      // g->syscallsp, g->syscallguard-StackGuard, g->syscallstack);
      runtime·throw("entersyscallblock");
  }

p = releasep();
handoffp(p);
if(g->isbackground)  // do not consider blocked scavenger for deadlock detection
  incidlelocked(1);

// Resave for traceback during blocked call.
save(runtime·getcallerpc(&dummy), runtime·getcallersp(&dummy));

g->stackguard0 = StackPreempt;  // see comment in entersyscall
m->locks--;
}

// The goroutine g exited its system call.
// Arrange for it to run on a cpu again.
// This is called only from the go syscall library, not
// from the low-level system calls used by the runtime.
#pragma textflag NOSPLIT
void
runtime·exitsyscall(void)
{
  m->locks++;  // see comment in entersyscall

  if(g->isbackground)  // do not consider blocked scavenger for deadlock detection
      incidlelocked(-1);

  if(exitsyscallfast()) {
      // There's a cpu for us, so we can run.
      m->p->syscalltick++;
      g->status = Grunning;
      // Garbage collector isn't running (since we are),
      // so okay to clear gcstack and gcsp.
      g->syscallstack = (uintptr)nil;
      g->syscallsp = (uintptr)nil;
      m->locks--;
      if(g->preempt) {
          // restore the preemption request in case we've cleared it in newstack
          g->stackguard0 = StackPreempt;
      } else {
          // otherwise restore the real stackguard, we've spoiled it in entersyscall/entersyscallblock
          g->stackguard0 = g->stackguard;
      }
      return;
  }

  m->locks--;

// Call the scheduler.
runtime·mcall(exitsyscall0);

// Scheduler returned, so we're allowed to run now.
// Delete the gcstack 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->syscallstack = (uintptr)nil;
g->syscallsp = (uintptr)nil;
m->p->syscalltick++;
}