pkg/sync/seqcount.go

// Copyright 2019 The gVisor Authors.
//
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package sync

import (
	"sync/atomic"
)

// SeqCount is a synchronization primitive for optimistic reader/writer
// synchronization in cases where readers can work with stale data and
// therefore do not need to block writers.
//
// Compared to sync/atomic.Value:
//
// - Mutation of SeqCount-protected data does not require memory allocation,
// whereas atomic.Value generally does. This is a significant advantage when
// writes are common.
//
// - Atomic reads of SeqCount-protected data require copying. This is a
// disadvantage when atomic reads are common.
//
// - SeqCount may be more flexible: correct use of SeqCount.ReadOk allows other
// operations to be made atomic with reads of SeqCount-protected data.
//
// - SeqCount is more cumbersome to use; atomic reads of SeqCount-protected
// data require instantiating function templates using go_generics (see
// seqatomic.go).
type SeqCount struct {
	// epoch is incremented by BeginWrite and EndWrite, such that epoch is odd
	// if a writer critical section is active, and a read from data protected
	// by this SeqCount is atomic iff epoch is the same even value before and
	// after the read.
	epoch uint32
}

// SeqCountEpoch tracks writer critical sections in a SeqCount.
type SeqCountEpoch uint32

// We assume that:
//
// - All functions in sync/atomic that perform a memory read are at least a
// read fence: memory reads before calls to such functions cannot be reordered
// after the call, and memory reads after calls to such functions cannot be
// reordered before the call, even if those reads do not use sync/atomic.
//
// - All functions in sync/atomic that perform a memory write are at least a
// write fence: memory writes before calls to such functions cannot be
// reordered after the call, and memory writes after calls to such functions
// cannot be reordered before the call, even if those writes do not use
// sync/atomic.
//
// As of this writing, the Go memory model completely fails to describe
// sync/atomic, but these properties are implied by
// https://groups.google.com/forum/#!topic/golang-nuts/7EnEhM3U7B8.

// BeginRead indicates the beginning of a reader critical section. Reader
// critical sections DO NOT BLOCK writer critical sections, so operations in a
// reader critical section MAY RACE with writer critical sections. Races are
// detected by ReadOk at the end of the reader critical section. Thus, the
// low-level structure of readers is generally:
//
//     for {
//         epoch := seq.BeginRead()
//         // do something idempotent with seq-protected data
//         if seq.ReadOk(epoch) {
//             break
//         }
//     }
//
// However, since reader critical sections may race with writer critical
// sections, the Go race detector will (accurately) flag data races in readers
// using this pattern. Most users of SeqCount will need to use the
// SeqAtomicLoad function template in seqatomic.go.
func (s *SeqCount) BeginRead() SeqCountEpoch {
	if epoch := atomic.LoadUint32(&s.epoch); epoch&1 == 0 {
		return SeqCountEpoch(epoch)
	}
	return s.beginReadSlow()
}

func (s *SeqCount) beginReadSlow() SeqCountEpoch {
	i := 0
	for {
		if canSpin(i) {
			i++
			doSpin()
		} else {
			goyield()
		}
		if epoch := atomic.LoadUint32(&s.epoch); epoch&1 == 0 {
			return SeqCountEpoch(epoch)
		}
	}
}

// ReadOk returns true if the reader critical section initiated by a previous
// call to BeginRead() that returned epoch did not race with any writer critical
// sections.
//
// ReadOk may be called any number of times during a reader critical section.
// Reader critical sections do not need to be explicitly terminated; the last
// call to ReadOk is implicitly the end of the reader critical section.
func (s *SeqCount) ReadOk(epoch SeqCountEpoch) bool {
	return atomic.LoadUint32(&s.epoch) == uint32(epoch)
}

// BeginWrite indicates the beginning of a writer critical section.
//
// SeqCount does not support concurrent writer critical sections; clients with
// concurrent writers must synchronize them using e.g. sync.Mutex.
func (s *SeqCount) BeginWrite() {
	if epoch := atomic.AddUint32(&s.epoch, 1); epoch&1 == 0 {
		panic("SeqCount.BeginWrite during writer critical section")
	}
}

// EndWrite ends the effect of a preceding BeginWrite.
func (s *SeqCount) EndWrite() {
	if epoch := atomic.AddUint32(&s.epoch, 1); epoch&1 != 0 {
		panic("SeqCount.EndWrite outside writer critical section")
	}
}