1 files changed, 152 insertions, 0 deletions
diff --git a/pkg/refsvfs2/refs_template.go b/pkg/refsvfs2/refs_template.go
new file mode 100644
index 000000000..8f50b4ee6
--- /dev/null
+++ b/pkg/refsvfs2/refs_template.go
@@ -0,0 +1,152 @@
+// 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. The "owner" template parameter is used in log messages to
+// indicate the type of reference-counted object that exhibited a reference
+// leak. As a result, structs that are embedded in other structs should not use
+// this template, since it will make tracking down leaks more difficult.
+package refs_template
+
+import (
+	"fmt"
+	"sync/atomic"
+
+	"gvisor.dev/gvisor/pkg/refsvfs2"
+)
+
+// enableLogging indicates whether reference-related events should be logged (with
+// stack traces). This is false by default and should only be set to true for
+// debugging purposes, as it can generate an extremely large amount of output
+// and drastically degrade performance.
+const enableLogging = false
+
+// T is the type of the reference counted object. It is only used to customize
+// debug output when leak checking.
+type T interface{}
+
+// obj 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 obj *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
+}
+
+// RefType implements refsvfs2.CheckedObject.RefType.
+func (r *Refs) RefType() string {
+	return fmt.Sprintf("%T", obj)[1:]
+}
+
+// LeakMessage implements refsvfs2.CheckedObject.LeakMessage.
+func (r *Refs) LeakMessage() string {
+	return fmt.Sprintf("[%s %p] reference count of %d instead of 0", r.RefType(), r, r.ReadRefs())
+}
+
+// LogRefs implements refsvfs2.CheckedObject.LogRefs.
+func (r *Refs) LogRefs() bool {
+	return enableLogging
+}
+
+// EnableLeakCheck enables reference leak checking on r.
+func (r *Refs) EnableLeakCheck() {
+	refsvfs2.Register(r)
+}
+
+// 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() {
+	v := atomic.AddInt64(&r.refCount, 1)
+	refsvfs2.LogIncRef(r, v+1)
+	if v <= 0 {
+		panic(fmt.Sprintf("Incrementing non-positive count %p on %s", r, r.RefType()))
+	}
+}
+
+// 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
+	if v := atomic.AddInt64(&r.refCount, speculativeRef); int32(v) < 0 {
+		// This object has already been freed.
+		atomic.AddInt64(&r.refCount, -speculativeRef)
+		return false
+	}
+
+	// Turn into a real reference.
+	v := atomic.AddInt64(&r.refCount, -speculativeRef+1)
+	refsvfs2.LogTryIncRef(r, v+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()) {
+	v := atomic.AddInt64(&r.refCount, -1)
+	refsvfs2.LogDecRef(r, v+1)
+	switch {
+	case v < -1:
+		panic(fmt.Sprintf("Decrementing non-positive ref count %p, owned by %s", r, r.RefType()))
+
+	case v == -1:
+		refsvfs2.Unregister(r)
+		// Call the destructor.
+		if destroy != nil {
+			destroy()
+		}
+	}
+}
+
+func (r *Refs) afterLoad() {
+	if r.ReadRefs() > 0 {
+		r.EnableLeakCheck()
+	}
+}