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package shm
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 ShmenableLogging = true
// 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 Shmobj *Shm
// Refs implements refs.RefCounter. It keeps a reference count using atomic
// operations and calls the destructor when the count reaches zero.
//
// NOTE: Do not introduce additional fields to the Refs struct. It is used by
// many filesystem objects, and we want to keep it as small as possible (i.e.,
// the same size as using an int64 directly) to avoid taking up extra cache
// space. In general, this template should not be extended at the cost of
// performance. If it does not offer enough flexibility for a particular object
// (example: b/187877947), we should implement the RefCounter/CheckedObject
// interfaces manually.
//
// +stateify savable
type ShmRefs 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
}
// InitRefs initializes r with one reference and, if enabled, activates leak
// checking.
func (r *ShmRefs) InitRefs() {
atomic.StoreInt64(&r.refCount, 1)
refsvfs2.Register(r)
}
// RefType implements refsvfs2.CheckedObject.RefType.
func (r *ShmRefs) RefType() string {
return fmt.Sprintf("%T", Shmobj)[1:]
}
// LeakMessage implements refsvfs2.CheckedObject.LeakMessage.
func (r *ShmRefs) 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 *ShmRefs) LogRefs() bool {
return ShmenableLogging
}
// ReadRefs returns the current number of references. The returned count is
// inherently racy and is unsafe to use without external synchronization.
func (r *ShmRefs) ReadRefs() int64 {
return atomic.LoadInt64(&r.refCount)
}
// IncRef implements refs.RefCounter.IncRef.
//
//go:nosplit
func (r *ShmRefs) IncRef() {
v := atomic.AddInt64(&r.refCount, 1)
if ShmenableLogging {
refsvfs2.LogIncRef(r, v)
}
if v <= 1 {
panic(fmt.Sprintf("Incrementing non-positive count %p on %s", r, r.RefType()))
}
}
// TryIncRef implements refs.TryRefCounter.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 *ShmRefs) TryIncRef() bool {
const speculativeRef = 1 << 32
if v := atomic.AddInt64(&r.refCount, speculativeRef); int32(v) == 0 {
atomic.AddInt64(&r.refCount, -speculativeRef)
return false
}
v := atomic.AddInt64(&r.refCount, -speculativeRef+1)
if ShmenableLogging {
refsvfs2.LogTryIncRef(r, v)
}
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 *ShmRefs) DecRef(destroy func()) {
v := atomic.AddInt64(&r.refCount, -1)
if ShmenableLogging {
refsvfs2.LogDecRef(r, v)
}
switch {
case v < 0:
panic(fmt.Sprintf("Decrementing non-positive ref count %p, owned by %s", r, r.RefType()))
case v == 0:
refsvfs2.Unregister(r)
if destroy != nil {
destroy()
}
}
}
func (r *ShmRefs) afterLoad() {
if r.ReadRefs() > 0 {
refsvfs2.Register(r)
}
}
|
