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// Copyright 2019 The gVisor Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// +build linux
package flipcall
import (
"encoding/json"
"fmt"
"runtime"
"sync/atomic"
"syscall"
"gvisor.dev/gvisor/pkg/abi/linux"
)
func (ep *Endpoint) futexConnect(req *ctrlHandshakeRequest) (ctrlHandshakeResponse, error) {
var resp ctrlHandshakeResponse
// Write the handshake request.
w := ep.NewWriter()
if err := json.NewEncoder(w).Encode(req); err != nil {
return resp, fmt.Errorf("error writing handshake request: %v", err)
}
*ep.dataLen() = w.Len()
// Exchange control with the server.
if err := ep.futexSwitchToPeer(); err != nil {
return resp, err
}
if err := ep.futexSwitchFromPeer(); err != nil {
return resp, err
}
// Read the handshake response.
respLen := atomic.LoadUint32(ep.dataLen())
if respLen > ep.dataCap {
return resp, fmt.Errorf("invalid handshake response length %d (maximum %d)", respLen, ep.dataCap)
}
if err := json.NewDecoder(ep.NewReader(respLen)).Decode(&resp); err != nil {
return resp, fmt.Errorf("error reading handshake response: %v", err)
}
return resp, nil
}
func (ep *Endpoint) futexSwitchToPeer() error {
// Update connection state to indicate that the peer should be active.
if !atomic.CompareAndSwapUint32(ep.connState(), ep.activeState, ep.inactiveState) {
return fmt.Errorf("unexpected connection state before FUTEX_WAKE: %v", atomic.LoadUint32(ep.connState()))
}
// Wake the peer's Endpoint.futexSwitchFromPeer().
if err := ep.futexWakeConnState(1); err != nil {
return fmt.Errorf("failed to FUTEX_WAKE peer Endpoint: %v", err)
}
return nil
}
func (ep *Endpoint) futexSwitchFromPeer() error {
for {
switch cs := atomic.LoadUint32(ep.connState()); cs {
case ep.activeState:
return nil
case ep.inactiveState:
// Continue to FUTEX_WAIT.
default:
return fmt.Errorf("unexpected connection state before FUTEX_WAIT: %v", cs)
}
if ep.isShutdownLocally() {
return shutdownError{}
}
if err := ep.futexWaitConnState(ep.inactiveState); err != nil {
return fmt.Errorf("failed to FUTEX_WAIT for peer Endpoint: %v", err)
}
}
}
func (ep *Endpoint) futexWakeConnState(numThreads int32) error {
if _, _, e := syscall.RawSyscall(syscall.SYS_FUTEX, ep.packet, linux.FUTEX_WAKE, uintptr(numThreads)); e != 0 {
return e
}
return nil
}
func (ep *Endpoint) futexWaitConnState(curState uint32) error {
_, _, e := syscall.Syscall6(syscall.SYS_FUTEX, ep.packet, linux.FUTEX_WAIT, uintptr(curState), 0, 0, 0)
if e != 0 && e != syscall.EAGAIN && e != syscall.EINTR {
return e
}
return nil
}
func yieldThread() {
syscall.Syscall(syscall.SYS_SCHED_YIELD, 0, 0, 0)
// The thread we're trying to yield to may be waiting for a Go runtime P.
// runtime.Gosched() will hand off ours if necessary.
runtime.Gosched()
}
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