1 files changed, 206 insertions, 0 deletions
diff --git a/pkg/tcpip/timer.go b/pkg/tcpip/timer.go
new file mode 100644
index 000000000..f1dd7c310
--- /dev/null
+++ b/pkg/tcpip/timer.go
@@ -0,0 +1,206 @@
+// Copyright 2020 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 tcpip
+
+import (
+	"time"
+
+	"gvisor.dev/gvisor/pkg/sync"
+)
+
+// jobInstance is a specific instance of Job.
+//
+// Different instances are created each time Job is scheduled so each timer has
+// its own earlyReturn signal. This is to address a bug when a Job is stopped
+// and reset in quick succession resulting in a timer instance's earlyReturn
+// signal being affected or seen by another timer instance.
+//
+// Consider the following sceneario where timer instances share a common
+// earlyReturn signal (T1 creates, stops and resets a Cancellable timer under a
+// lock L; T2, T3, T4 and T5 are goroutines that handle the first (A), second
+// (B), third (C), and fourth (D) instance of the timer firing, respectively):
+//   T1: Obtain L
+//   T1: Create a new Job w/ lock L (create instance A)
+//   T2: instance A fires, blocked trying to obtain L.
+//   T1: Attempt to stop instance A (set earlyReturn = true)
+//   T1: Schedule timer (create instance B)
+//   T3: instance B fires, blocked trying to obtain L.
+//   T1: Attempt to stop instance B (set earlyReturn = true)
+//   T1: Schedule timer (create instance C)
+//   T4: instance C fires, blocked trying to obtain L.
+//   T1: Attempt to stop instance C (set earlyReturn = true)
+//   T1: Schedule timer (create instance D)
+//   T5: instance D fires, blocked trying to obtain L.
+//   T1: Release L
+//
+// Now that T1 has released L, any of the 4 timer instances can take L and
+// check earlyReturn. If the timers simply check earlyReturn and then do
+// nothing further, then instance D will never early return even though it was
+// not requested to stop. If the timers reset earlyReturn before early
+// returning, then all but one of the timers will do work when only one was
+// expected to. If Job resets earlyReturn when resetting, then all the timers
+// will fire (again, when only one was expected to).
+//
+// To address the above concerns the simplest solution was to give each timer
+// its own earlyReturn signal.
+type jobInstance struct {
+	timer Timer
+
+	// Used to inform the timer to early return when it gets stopped while the
+	// lock the timer tries to obtain when fired is held (T1 is a goroutine that
+	// tries to cancel the timer and T2 is the goroutine that handles the timer
+	// firing):
+	//   T1: Obtain the lock, then call Cancel()
+	//   T2: timer fires, and gets blocked on obtaining the lock
+	//   T1: Releases lock
+	//   T2: Obtains lock does unintended work
+	//
+	// To resolve this, T1 will check to see if the timer already fired, and
+	// inform the timer using earlyReturn to return early so that once T2 obtains
+	// the lock, it will see that it is set to true and do nothing further.
+	earlyReturn *bool
+}
+
+// stop stops the job instance j from firing if it hasn't fired already. If it
+// has fired and is blocked at obtaining the lock, earlyReturn will be set to
+// true so that it will early return when it obtains the lock.
+func (j *jobInstance) stop() {
+	if j.timer != nil {
+		j.timer.Stop()
+		*j.earlyReturn = true
+	}
+}
+
+// Job represents some work that can be scheduled for execution. The work can
+// be safely cancelled when it fires at the same time some "related work" is
+// being done.
+//
+// The term "related work" is defined as some work that needs to be done while
+// holding some lock that the timer must also hold while doing some work.
+//
+// Note, it is not safe to copy a Job as its timer instance creates
+// a closure over the address of the Job.
+type Job struct {
+	_ sync.NoCopy
+
+	// The clock used to schedule the backing timer
+	clock Clock
+
+	// The active instance of a cancellable timer.
+	instance jobInstance
+
+	// locker is the lock taken by the timer immediately after it fires and must
+	// be held when attempting to stop the timer.
+	//
+	// Must never change after being assigned.
+	locker sync.Locker
+
+	// fn is the function that will be called when a timer fires and has not been
+	// signaled to early return.
+	//
+	// fn MUST NOT attempt to lock locker.
+	//
+	// Must never change after being assigned.
+	fn func()
+}
+
+// Cancel prevents the Job from executing if it has not executed already.
+//
+// Cancel requires appropriate locking to be in place for any resources managed
+// by the Job. If the Job is blocked on obtaining the lock when Cancel is
+// called, it will early return.
+//
+// Note, t will be modified.
+//
+// j.locker MUST be locked.
+func (j *Job) Cancel() {
+	j.instance.stop()
+
+	// Nothing to do with the stopped instance anymore.
+	j.instance = jobInstance{}
+}
+
+// Schedule schedules the Job for execution after duration d. This can be
+// called on cancelled or completed Jobs to schedule them again.
+//
+// Schedule should be invoked only on unscheduled, cancelled, or completed
+// Jobs. To be safe, callers should always call Cancel before calling Schedule.
+//
+// Note, j will be modified.
+func (j *Job) Schedule(d time.Duration) {
+	// Create a new instance.
+	earlyReturn := false
+
+	// Capture the locker so that updating the timer does not cause a data race
+	// when a timer fires and tries to obtain the lock (read the timer's locker).
+	locker := j.locker
+	j.instance = jobInstance{
+		timer: j.clock.AfterFunc(d, func() {
+			locker.Lock()
+			defer locker.Unlock()
+
+			if earlyReturn {
+				// If we reach this point, it means that the timer fired while another
+				// goroutine called Cancel while it had the lock. Simply return here
+				// and do nothing further.
+				earlyReturn = false
+				return
+			}
+
+			j.fn()
+		}),
+		earlyReturn: &earlyReturn,
+	}
+}
+
+// NewJob returns a new Job that can be used to schedule f to run in its own
+// gorountine. l will be locked before calling f then unlocked after f returns.
+//
+//  var clock tcpip.StdClock
+//  var mu sync.Mutex
+//  message := "foo"
+//  job := tcpip.NewJob(&clock, &mu, func() {
+//    fmt.Println(message)
+//  })
+//  job.Schedule(time.Second)
+//
+//  mu.Lock()
+//  message = "bar"
+//  mu.Unlock()
+//
+//  // Output: bar
+//
+// f MUST NOT attempt to lock l.
+//
+// l MUST be locked prior to calling the returned job's Cancel().
+//
+//  var clock tcpip.StdClock
+//  var mu sync.Mutex
+//  message := "foo"
+//  job := tcpip.NewJob(&clock, &mu, func() {
+//    fmt.Println(message)
+//  })
+//  job.Schedule(time.Second)
+//
+//  mu.Lock()
+//  job.Cancel()
+//  mu.Unlock()
+func NewJob(c Clock, l sync.Locker, f func()) *Job {
+	return &Job{
+		clock:  c,
+		locker: l,
+		fn:     f,
+	}
+}