diff options
Diffstat (limited to 'pkg/sentry/kernel/session_refs.go')
-rw-r--r-- | pkg/sentry/kernel/session_refs.go | 118 |
1 files changed, 118 insertions, 0 deletions
diff --git a/pkg/sentry/kernel/session_refs.go b/pkg/sentry/kernel/session_refs.go new file mode 100644 index 000000000..f2e1bb797 --- /dev/null +++ b/pkg/sentry/kernel/session_refs.go @@ -0,0 +1,118 @@ +package kernel + +import ( + "fmt" + "runtime" + "sync/atomic" + + "gvisor.dev/gvisor/pkg/log" + refs_vfs1 "gvisor.dev/gvisor/pkg/refs" +) + +// 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 SessionownerType *Session + +// 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. +// +// TODO(gvisor.dev/issue/1486): Store stack traces when leak check is enabled in +// a map with 16-bit hashes, and store the hash in the top 16 bits of refCount. +// This will allow us to add stack trace information to the leak messages +// without growing the size of Refs. +// +// +stateify savable +type SessionRefs 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 *SessionRefs) 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("%sRefs %p owned by %T garbage collected with ref count of %d (want 0)", note, r, SessionownerType, n) + } +} + +// EnableLeakCheck checks for reference leaks when Refs gets garbage collected. +func (r *SessionRefs) EnableLeakCheck() { + if refs_vfs1.GetLeakMode() != refs_vfs1.NoLeakChecking { + runtime.SetFinalizer(r, (*SessionRefs).finalize) + } +} + +// ReadRefs returns the current number of references. The returned count is +// inherently racy and is unsafe to use without external synchronization. +func (r *SessionRefs) ReadRefs() int64 { + + return atomic.LoadInt64(&r.refCount) + 1 +} + +// IncRef implements refs.RefCounter.IncRef. +// +//go:nosplit +func (r *SessionRefs) IncRef() { + if v := atomic.AddInt64(&r.refCount, 1); v <= 0 { + panic(fmt.Sprintf("Incrementing non-positive ref count %p owned by %T", r, SessionownerType)) + } +} + +// 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 *SessionRefs) TryIncRef() bool { + const speculativeRef = 1 << 32 + v := atomic.AddInt64(&r.refCount, speculativeRef) + if int32(v) < 0 { + + atomic.AddInt64(&r.refCount, -speculativeRef) + return false + } + + 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 *SessionRefs) DecRef(destroy func()) { + switch v := atomic.AddInt64(&r.refCount, -1); { + case v < -1: + panic(fmt.Sprintf("Decrementing non-positive ref count %p, owned by %T", r, SessionownerType)) + + case v == -1: + + if destroy != nil { + destroy() + } + } +} |