From f4a4cde7ddc19b1e5f434d5e15f089af4f6f0f15 Mon Sep 17 00:00:00 2001
From: Kevin Krakauer <krakauer@google.com>
Date: Tue, 30 May 2023 12:05:24 -0700
Subject: [PATCH] netstack: remove mutex from standard clock

We can avoid locking altogether by saving the monotonic offset only during save
rather than every time we return a time.

PiperOrigin-RevId: 536471006
---
 pkg/tcpip/stdclock.go       | 25 ++++---------------------
 pkg/tcpip/stdclock_state.go |  9 +++++----
 2 files changed, 9 insertions(+), 25 deletions(-)

diff --git a/pkg/tcpip/stdclock.go b/pkg/tcpip/stdclock.go
index 371da2f40..cc3397ca9 100644
--- a/pkg/tcpip/stdclock.go
+++ b/pkg/tcpip/stdclock.go
@@ -17,8 +17,6 @@ package tcpip
 import (
 	"fmt"
 	"time"
-
-	"gvisor.dev/gvisor/pkg/sync"
 )
 
 // stdClock implements Clock with the time package.
@@ -57,14 +55,9 @@ type stdClock struct {
 
 	// monotonicOffset is the offset applied to the calculated monotonic time.
 	//
-	// monotonicOffset is assigned maxMonotonic after restore so that the
-	// monotonic time will continue from where it "left off" before saving as part
-	// of S/R.
-	monotonicOffset MonotonicTime `state:"nosave"`
-
-	// monotonicMU protects maxMonotonic.
-	monotonicMU  sync.Mutex `state:"nosave"`
-	maxMonotonic MonotonicTime
+	// monotonicOffset is assigned after restore so that the monotonic time
+	// will continue from where it "left off" before saving as part of S/R.
+	monotonicOffset MonotonicTime
 }
 
 // NewStdClock returns an instance of a clock that uses the time package.
@@ -88,17 +81,7 @@ func (s *stdClock) NowMonotonic() MonotonicTime {
 		panic(fmt.Sprintf("got negative duration = %s since base time = %s", sinceBase, s.baseTime))
 	}
 
-	monotonicValue := s.monotonicOffset.Add(sinceBase)
-
-	s.monotonicMU.Lock()
-	defer s.monotonicMU.Unlock()
-
-	// Monotonic time values must never decrease.
-	if s.maxMonotonic.Before(monotonicValue) {
-		s.maxMonotonic = monotonicValue
-	}
-
-	return s.maxMonotonic
+	return s.monotonicOffset.Add(sinceBase)
 }
 
 // AfterFunc implements Clock.AfterFunc.
diff --git a/pkg/tcpip/stdclock_state.go b/pkg/tcpip/stdclock_state.go
index 795db9181..25be1755f 100644
--- a/pkg/tcpip/stdclock_state.go
+++ b/pkg/tcpip/stdclock_state.go
@@ -16,11 +16,12 @@ package tcpip
 
 import "time"
 
+// beforeSave is invoked by stateify.
+func (s *stdClock) beforeSave() {
+	s.monotonicOffset = s.NowMonotonic()
+}
+
 // afterLoad is invoked by stateify.
 func (s *stdClock) afterLoad() {
 	s.baseTime = time.Now()
-
-	s.monotonicMU.Lock()
-	defer s.monotonicMU.Unlock()
-	s.monotonicOffset = s.maxMonotonic
 }