1 files changed, 141 insertions, 0 deletions
diff --git a/pkg/tcpip/transport/tcp/timer.go b/pkg/tcpip/transport/tcp/timer.go
new file mode 100644
index 000000000..fc1c7cbd2
--- /dev/null
+++ b/pkg/tcpip/transport/tcp/timer.go
@@ -0,0 +1,141 @@
+// 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 (
+	"time"
+
+	"gvisor.googlesource.com/gvisor/pkg/sleep"
+)
+
+type timerState int
+
+const (
+	timerStateDisabled timerState = iota
+	timerStateEnabled
+	timerStateOrphaned
+)
+
+// timer is a timer implementation that reduces the interactions with the
+// runtime 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 runtime 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 is the current state of the timer, it can be one of the
+	// following values:
+	//     disabled - the timer is disabled.
+	//     orphaned - the timer is disabled, but the runtime timer is
+	//                enabled, which means that it will evetually cause a
+	//                spurious wake (unless it gets enabled again before
+	//                then).
+	//     enabled  - the timer is enabled, but the runtime timer may be set
+	//                to an earlier expiration time due to a previous
+	//                orphaned state.
+	state timerState
+
+	// target is the expiration time of the current timer. It is only
+	// meaningful in the enabled state.
+	target time.Time
+
+	// runtimeTarget is the expiration time of the runtime timer. It is
+	// meaningful in the enabled and orphaned states.
+	runtimeTarget time.Time
+
+	// timer is the runtime timer used to wait on.
+	timer *time.Timer
+}
+
+// init initializes the timer. Once it expires, it the given waker will be
+// asserted.
+func (t *timer) init(w *sleep.Waker) {
+	t.state = timerStateDisabled
+
+	// Initialize a runtime timer that will assert the waker, then
+	// immediately stop it.
+	t.timer = time.AfterFunc(time.Hour, func() {
+		w.Assert()
+	})
+	t.timer.Stop()
+}
+
+// cleanup frees all resources associated with the timer.
+func (t *timer) cleanup() {
+	t.timer.Stop()
+}
+
+// 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 := time.Now()
+	if now.Before(t.target) {
+		t.runtimeTarget = 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 = time.Now().Add(d)
+
+	// Check if we need to set the runtime timer.
+	if t.state == timerStateDisabled || t.target.Before(t.runtimeTarget) {
+		t.runtimeTarget = t.target
+		t.timer.Reset(d)
+	}
+
+	t.state = timerStateEnabled
+}