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// Copyright 2018 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.
package tcp
import (
"math"
"time"
"gvisor.dev/gvisor/pkg/sleep"
"gvisor.dev/gvisor/pkg/tcpip"
)
type timerState int
const (
// The timer is disabled.
timerStateDisabled timerState = iota
// The timer is enabled, but the clock timer may be set to an earlier
// expiration time due to a previous orphaned state.
timerStateEnabled
// The timer is disabled, but the clock timer is enabled, which means that
// it will cause a spurious wakeup unless the timer is enabled before the
// clock timer fires.
timerStateOrphaned
)
// timer is a timer implementation that reduces the interactions with the
// clock timer infrastructure by letting timers run (and potentially
// eventually expire) even if they are stopped. It makes it cheaper to
// disable/reenable timers at the expense of spurious wakes. This is useful for
// cases when the same timer is disabled/reenabled repeatedly with relatively
// long timeouts farther into the future.
//
// TCP retransmit timers benefit from this because they the timeouts are long
// (currently at least 200ms), and get disabled when acks are received, and
// reenabled when new pending segments are sent.
//
// It is advantageous to avoid interacting with the clock because it acquires
// a global mutex and performs O(log n) operations, where n is the global number
// of timers, whenever a timer is enabled or disabled, and may make a syscall.
//
// This struct is thread-compatible.
type timer struct {
state timerState
clock tcpip.Clock
// target is the expiration time of the current timer. It is only
// meaningful in the enabled state.
target tcpip.MonotonicTime
// clockTarget is the expiration time of the clock timer. It is
// meaningful in the enabled and orphaned states.
clockTarget tcpip.MonotonicTime
// timer is the clock timer used to wait on.
timer tcpip.Timer
}
// init initializes the timer. Once it expires, it the given waker will be
// asserted.
func (t *timer) init(clock tcpip.Clock, w *sleep.Waker) {
t.state = timerStateDisabled
t.clock = clock
// Initialize a clock timer that will assert the waker, then
// immediately stop it.
t.timer = t.clock.AfterFunc(math.MaxInt64, func() {
w.Assert()
})
t.timer.Stop()
}
// cleanup frees all resources associated with the timer.
func (t *timer) cleanup() {
if t.timer == nil {
// No cleanup needed.
return
}
t.timer.Stop()
*t = timer{}
}
// checkExpiration checks if the given timer has actually expired, it should be
// called whenever a sleeper wakes up due to the waker being asserted, and is
// used to check if it's a supurious wake (due to a previously orphaned timer)
// or a legitimate one.
func (t *timer) checkExpiration() bool {
// Transition to fully disabled state if we're just consuming an
// orphaned timer.
if t.state == timerStateOrphaned {
t.state = timerStateDisabled
return false
}
// The timer is enabled, but it may have expired early. Check if that's
// the case, and if so, reset the runtime timer to the correct time.
now := t.clock.NowMonotonic()
if now.Before(t.target) {
t.clockTarget = t.target
t.timer.Reset(t.target.Sub(now))
return false
}
// The timer has actually expired, disable it for now and inform the
// caller.
t.state = timerStateDisabled
return true
}
// disable disables the timer, leaving it in an orphaned state if it wasn't
// already disabled.
func (t *timer) disable() {
if t.state != timerStateDisabled {
t.state = timerStateOrphaned
}
}
// enabled returns true if the timer is currently enabled, false otherwise.
func (t *timer) enabled() bool {
return t.state == timerStateEnabled
}
// enable enables the timer, programming the runtime timer if necessary.
func (t *timer) enable(d time.Duration) {
t.target = t.clock.NowMonotonic().Add(d)
// Check if we need to set the runtime timer.
if t.state == timerStateDisabled || t.target.Before(t.clockTarget) {
t.clockTarget = t.target
t.timer.Reset(d)
}
t.state = timerStateEnabled
}
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