3 files changed, 197 insertions, 0 deletions
diff --git a/pkg/refs_vfs2/BUILD b/pkg/refs_vfs2/BUILD
new file mode 100644
index 000000000..7f180c7bd
--- /dev/null
+++ b/pkg/refs_vfs2/BUILD
@@ -0,0 +1,28 @@
+load("//tools:defs.bzl", "go_library")
+load("//tools/go_generics:defs.bzl", "go_template")
+
+package(licenses = ["notice"])
+
+go_template(
+    name = "refs_template",
+    srcs = [
+        "refs_template.go",
+    ],
+    types = [
+        "T",
+    ],
+    visibility = ["//pkg/sentry:internal"],
+    deps = [
+        "//pkg/log",
+        "//pkg/refs",
+    ],
+)
+
+go_library(
+    name = "refs",
+    srcs = [
+        "refs.go",
+    ],
+    visibility = ["//pkg/sentry:internal"],
+    deps = ["//pkg/context"],
+)
diff --git a/pkg/refs_vfs2/refs.go b/pkg/refs_vfs2/refs.go
new file mode 100644
index 000000000..ee01b17b0
--- /dev/null
+++ b/pkg/refs_vfs2/refs.go
@@ -0,0 +1,36 @@
+// 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 refs defines an interface for a reference-counted object.
+package refs
+
+import (
+	"gvisor.dev/gvisor/pkg/context"
+)
+
+// RefCounter is the interface to be implemented by objects that are reference
+// counted.
+type RefCounter interface {
+	// IncRef increments the reference counter on the object.
+	IncRef()
+
+	// DecRef decrements the object's reference count. Users of refs_template.Refs
+	// may specify a destructor to be called once the reference count reaches zero.
+	DecRef(ctx context.Context)
+
+	// TryIncRef attempts to increment the reference count, but may fail if all
+	// references have already been dropped, in which case it returns false. If
+	// true is returned, then a valid reference is now held on the object.
+	TryIncRef() bool
+}
diff --git a/pkg/refs_vfs2/refs_template.go b/pkg/refs_vfs2/refs_template.go
new file mode 100644
index 000000000..3e5b458c7
--- /dev/null
+++ b/pkg/refs_vfs2/refs_template.go
@@ -0,0 +1,133 @@
+// 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 refs_template defines a template that can be used by reference counted
+// objects.
+package refs_template
+
+import (
+	"runtime"
+	"sync/atomic"
+
+	"gvisor.dev/gvisor/pkg/log"
+	refs_vfs1 "gvisor.dev/gvisor/pkg/refs"
+)
+
+// T is the type of the reference counted object. It is only used to customize
+// debug output when leak checking.
+type T interface{}
+
+// ownerType is used to customize logging. Note that we use a pointer to T so
+// that we do not copy the entire object when passed as a format parameter.
+var ownerType *T
+
+// Refs implements refs.RefCounter. It keeps a reference count using atomic
+// operations and calls the destructor when the count reaches zero.
+//
+// Note that the number of references is actually refCount + 1 so that a default
+// zero-value Refs object contains one reference.
+//
+// +stateify savable
+type Refs struct {
+	// refCount is composed of two fields:
+	//
+	//	[32-bit speculative references]:[32-bit real references]
+	//
+	// Speculative references are used for TryIncRef, to avoid a CompareAndSwap
+	// loop. See IncRef, DecRef and TryIncRef for details of how these fields are
+	// used.
+	refCount int64
+}
+
+func (r *Refs) finalize() {
+	var note string
+	switch refs_vfs1.GetLeakMode() {
+	case refs_vfs1.NoLeakChecking:
+		return
+	case refs_vfs1.UninitializedLeakChecking:
+		note = "(Leak checker uninitialized): "
+	}
+	if n := r.ReadRefs(); n != 0 {
+		log.Warningf("%sAtomicRefCount %p owned by %T garbage collected with ref count of %d (want 0)", note, r, ownerType, n)
+	}
+}
+
+// EnableLeakCheck checks for reference leaks when Refs gets garbage collected.
+func (r *Refs) EnableLeakCheck() {
+	if refs_vfs1.GetLeakMode() != refs_vfs1.NoLeakChecking {
+		runtime.SetFinalizer(r, (*Refs).finalize)
+	}
+}
+
+// ReadRefs returns the current number of references. The returned count is
+// inherently racy and is unsafe to use without external synchronization.
+func (r *Refs) ReadRefs() int64 {
+	// Account for the internal -1 offset on refcounts.
+	return atomic.LoadInt64(&r.refCount) + 1
+}
+
+// IncRef implements refs.RefCounter.IncRef.
+//
+//go:nosplit
+func (r *Refs) IncRef() {
+	if v := atomic.AddInt64(&r.refCount, 1); v <= 0 {
+		panic("Incrementing non-positive ref count")
+	}
+}
+
+// TryIncRef implements refs.RefCounter.TryIncRef.
+//
+// To do this safely without a loop, a speculative reference is first acquired
+// on the object. This allows multiple concurrent TryIncRef calls to distinguish
+// other TryIncRef calls from genuine references held.
+//
+//go:nosplit
+func (r *Refs) TryIncRef() bool {
+	const speculativeRef = 1 << 32
+	v := atomic.AddInt64(&r.refCount, speculativeRef)
+	if int32(v) < 0 {
+		// This object has already been freed.
+		atomic.AddInt64(&r.refCount, -speculativeRef)
+		return false
+	}
+
+	// Turn into a real reference.
+	atomic.AddInt64(&r.refCount, -speculativeRef+1)
+	return true
+}
+
+// DecRef implements refs.RefCounter.DecRef.
+//
+// Note that speculative references are counted here. Since they were added
+// prior to real references reaching zero, they will successfully convert to
+// real references. In other words, we see speculative references only in the
+// following case:
+//
+//	A: TryIncRef [speculative increase => sees non-negative references]
+//	B: DecRef [real decrease]
+//	A: TryIncRef [transform speculative to real]
+//
+//go:nosplit
+func (r *Refs) DecRef(destroy func()) {
+	switch v := atomic.AddInt64(&r.refCount, -1); {
+	case v < -1:
+		panic("Decrementing non-positive ref count")
+
+	case v == -1:
+		// Call the destructor.
+		if destroy != nil {
+			destroy()
+		}
+	}
+}