Allow use of SeqAtomic with pointer-containing types.

Per runtime.memmove, pointers are always copied atomically, as this is required
by the GC. (Also, the init() safety check doesn't work because it gets renamed
to <prefix>init() by template instantiation.)

PiperOrigin-RevId: 345800302

pkg/sync/seqatomic_unsafe.go

@@ -8,20 +8,12 @@
 package template
 
 import (
-	"fmt"
-	"reflect"
-	"strings"
 	"unsafe"
 
 	"gvisor.dev/gvisor/pkg/sync"
 )
 
 // Value is a required type parameter.
-//
-// Value must not contain any pointers, including interface objects, function
-// objects, slices, maps, channels, unsafe.Pointer, and arrays or structs
-// containing any of the above. An init() function will panic if this property
-// does not hold.
 type Value struct{}
 
 // SeqAtomicLoad returns a copy of *ptr, ensuring that the read does not race
@@ -55,12 +47,3 @@ func SeqAtomicTryLoad(seq *sync.SeqCount, epoch sync.SeqCountEpoch, ptr *Value)
 	ok = seq.ReadOk(epoch)
 	return
 }
-
-func init() {
-	var val Value
-	typ := reflect.TypeOf(val)
-	name := typ.Name()
-	if ptrs := sync.PointersInType(typ, name); len(ptrs) != 0 {
-		panic(fmt.Sprintf("SeqAtomicLoad<%s> is invalid since values %s of type %s contain pointers:\n%s", typ, name, typ, strings.Join(ptrs, "\n")))
-	}
-}

pkg/sync/seqcount.go

@@ -6,8 +6,6 @@
 package sync
 
 import (
-	"fmt"
-	"reflect"
 	"sync/atomic"
 )
 
@@ -27,9 +25,6 @@ import (
 // - SeqCount may be more flexible: correct use of SeqCount.ReadOk allows other
 // operations to be made atomic with reads of SeqCount-protected data.
 //
-// - SeqCount may be less flexible: as of this writing, SeqCount-protected data
-// cannot include pointers.
-//
 // - SeqCount is more cumbersome to use; atomic reads of SeqCount-protected
 // data require instantiating function templates using go_generics (see
 // seqatomic.go).
@@ -128,32 +123,3 @@ func (s *SeqCount) EndWrite() {
 		panic("SeqCount.EndWrite outside writer critical section")
 	}
 }
-
-// PointersInType returns a list of pointers reachable from values named
-// valName of the given type.
-//
-// PointersInType is not exhaustive, but it is guaranteed that if typ contains
-// at least one pointer, then PointersInTypeOf returns a non-empty list.
-func PointersInType(typ reflect.Type, valName string) []string {
-	switch kind := typ.Kind(); kind {
-	case reflect.Bool, reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64, reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr, reflect.Float32, reflect.Float64, reflect.Complex64, reflect.Complex128:
-		return nil
-
-	case reflect.Chan, reflect.Func, reflect.Interface, reflect.Map, reflect.Ptr, reflect.Slice, reflect.String, reflect.UnsafePointer:
-		return []string{valName}
-
-	case reflect.Array:
-		return PointersInType(typ.Elem(), valName+"[]")
-
-	case reflect.Struct:
-		var ptrs []string
-		for i, n := 0, typ.NumField(); i < n; i++ {
-			field := typ.Field(i)
-			ptrs = append(ptrs, PointersInType(field.Type, fmt.Sprintf("%s.%s", valName, field.Name))...)
-		}
-		return ptrs
-
-	default:
-		return []string{fmt.Sprintf("%s (of type %s with unknown kind %s)", valName, typ, kind)}
-	}
-}

pkg/sync/seqcount_test.go

@@ -6,7 +6,6 @@
 package sync
 
 import (
-	"reflect"
 	"testing"
 	"time"
 )
@@ -99,55 +98,3 @@ func BenchmarkSeqCountReadUncontended(b *testing.B) {
 		}
 	})
 }
-
-func TestPointersInType(t *testing.T) {
-	for _, test := range []struct {
-		name string // used for both test and value name
-		val  interface{}
-		ptrs []string
-	}{
-		{
-			name: "EmptyStruct",
-			val:  struct{}{},
-		},
-		{
-			name: "Int",
-			val:  int(0),
-		},
-		{
-			name: "MixedStruct",
-			val: struct {
-				b             bool
-				I             int
-				ExportedPtr   *struct{}
-				unexportedPtr *struct{}
-				arr           [2]int
-				ptrArr        [2]*int
-				nestedStruct  struct {
-					nestedNonptr int
-					nestedPtr    *int
-				}
-				structArr [1]struct {
-					nonptr int
-					ptr    *int
-				}
-			}{},
-			ptrs: []string{
-				"MixedStruct.ExportedPtr",
-				"MixedStruct.unexportedPtr",
-				"MixedStruct.ptrArr[]",
-				"MixedStruct.nestedStruct.nestedPtr",
-				"MixedStruct.structArr[].ptr",
-			},
-		},
-	} {
-		t.Run(test.name, func(t *testing.T) {
-			typ := reflect.TypeOf(test.val)
-			ptrs := PointersInType(typ, test.name)
-			t.Logf("Found pointers: %v", ptrs)
-			if (len(ptrs) != 0 || len(test.ptrs) != 0) && !reflect.DeepEqual(ptrs, test.ptrs) {
-				t.Errorf("Got %v, wanted %v", ptrs, test.ptrs)
-			}
-		})
-	}
-}