atomicbitops package cleanups

- Redocument memory ordering from "no ordering" to "acquire-release". (No
  functional change: both LOCK WHATEVER on x86, and LDAXR/STLXR loops on ARM64,
  already have this property.)

- Remove IncUnlessZeroInt32 and DecUnlessOneInt32, which were only faster than
  the equivalent loops using sync/atomic before the Go compiler inlined
  non-unsafe.Pointer atomics many releases ago.

PiperOrigin-RevId: 295811743

2 files changed, 27 insertions, 7 deletions

diff --git a/pkg/sentry/mm/address_space.go b/pkg/sentry/mm/address_space.go
index e58a63deb..94d39af60 100644
--- a/pkg/sentry/mm/address_space.go
+++ b/pkg/sentry/mm/address_space.go
@@ -18,7 +18,6 @@ import (
 	"fmt"
 	"sync/atomic"
 
-	"gvisor.dev/gvisor/pkg/atomicbitops"
 	"gvisor.dev/gvisor/pkg/sentry/platform"
 	"gvisor.dev/gvisor/pkg/usermem"
 )
@@ -42,8 +41,15 @@ func (mm *MemoryManager) AddressSpace() platform.AddressSpace {
 func (mm *MemoryManager) Activate() error {
 	// Fast path: the MemoryManager already has an active
 	// platform.AddressSpace, and we just need to indicate that we need it too.
-	if atomicbitops.IncUnlessZeroInt32(&mm.active) {
-		return nil
+	for {
+		active := atomic.LoadInt32(&mm.active)
+		if active == 0 {
+			// Fall back to the slow path.
+			break
+		}
+		if atomic.CompareAndSwapInt32(&mm.active, active, active+1) {
+			return nil
+		}
 	}
 
 	for {
@@ -118,8 +124,15 @@ func (mm *MemoryManager) Activate() error {
 func (mm *MemoryManager) Deactivate() {
 	// Fast path: this is not the last goroutine to deactivate the
 	// MemoryManager.
-	if atomicbitops.DecUnlessOneInt32(&mm.active) {
-		return
+	for {
+		active := atomic.LoadInt32(&mm.active)
+		if active == 1 {
+			// Fall back to the slow path.
+			break
+		}
+		if atomic.CompareAndSwapInt32(&mm.active, active, active-1) {
+			return
+		}
 	}
 
 	mm.activeMu.Lock()
diff --git a/pkg/sentry/mm/lifecycle.go b/pkg/sentry/mm/lifecycle.go
index 47b8fbf43..3c263ebaa 100644
--- a/pkg/sentry/mm/lifecycle.go
+++ b/pkg/sentry/mm/lifecycle.go
@@ -18,7 +18,6 @@ import (
 	"fmt"
 	"sync/atomic"
 
-	"gvisor.dev/gvisor/pkg/atomicbitops"
 	"gvisor.dev/gvisor/pkg/context"
 	"gvisor.dev/gvisor/pkg/sentry/arch"
 	"gvisor.dev/gvisor/pkg/sentry/limits"
@@ -229,7 +228,15 @@ func (mm *MemoryManager) Fork(ctx context.Context) (*MemoryManager, error) {
 // IncUsers increments mm's user count and returns true. If the user count is
 // already 0, IncUsers does nothing and returns false.
 func (mm *MemoryManager) IncUsers() bool {
-	return atomicbitops.IncUnlessZeroInt32(&mm.users)
+	for {
+		users := atomic.LoadInt32(&mm.users)
+		if users == 0 {
+			return false
+		}
+		if atomic.CompareAndSwapInt32(&mm.users, users, users+1) {
+			return true
+		}
+	}
 }
 
 // DecUsers decrements mm's user count. If the user count reaches 0, all