minio/pkg/dsync/drwmutex.go at ilm · ggbg/minio

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 * Minio Cloud Storage, (C) 2016 Minio, Inc.
 * 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 dsync
	"math/rand"
	"github.com/minio/minio/pkg/console"
// Indicator if logging is enabled.
var dsyncLog bool
func init() {
	// Check for MINIO_DSYNC_TRACE env variable, if set logging will be enabled for failed REST operations.
	dsyncLog = os.Getenv("MINIO_DSYNC_TRACE") == "1"
func log(format string, data ...interface{}) {
	if dsyncLog {
		console.Printf(format, data...)
// DRWMutexAcquireTimeout - tolerance limit to wait for lock acquisition before.
const DRWMutexAcquireTimeout = 1 * time.Second // 1 second.
const drwMutexInfinite = 1<<63 - 1
// A DRWMutex is a distributed mutual exclusion lock.
type DRWMutex struct {
	Names        []string
	writeLocks   []string   // Array of nodes that granted a write lock
	readersLocks [][]string // Array of array of nodes that granted reader locks
	m            sync.Mutex // Mutex to prevent multiple simultaneous locks from this node
	clnt         *Dsync
// Granted - represents a structure of a granted lock.
type Granted struct {
	index   int
	lockUID string // Locked if set with UID string, unlocked if empty
func (g *Granted) isLocked() bool {
	return isLocked(g.lockUID)
func isLocked(uid string) bool {
	return len(uid) > 0
// NewDRWMutex - initializes a new dsync RW mutex.
func NewDRWMutex(clnt *Dsync, names ...string) *DRWMutex {
	restClnts, _ := clnt.GetLockers()
	return &DRWMutex{
		writeLocks: make([]string, len(restClnts)),
		Names:      names,
		clnt:       clnt,
// Lock holds a write lock on dm.
// If the lock is already in use, the calling go routine
// blocks until the mutex is available.
func (dm *DRWMutex) Lock(id, source string) {
	isReadLock := false
	dm.lockBlocking(context.Background(), id, source, isReadLock, Options{
		Timeout: drwMutexInfinite,
// Options lock options.
type Options struct {
	Timeout time.Duration
// GetLock tries to get a write lock on dm before the timeout elapses.
// If the lock is already in use, the calling go routine
// blocks until either the mutex becomes available and return success or
// more time has passed than the timeout value and return false.
func (dm *DRWMutex) GetLock(ctx context.Context, id, source string, opts Options) (locked bool) {
	isReadLock := false
	return dm.lockBlocking(ctx, id, source, isReadLock, opts)
// RLock holds a read lock on dm.
// If one or more read locks are already in use, it will grant another lock.
// Otherwise the calling go routine blocks until the mutex is available.
func (dm *DRWMutex) RLock(id, source string) {
	isReadLock := true
	dm.lockBlocking(context.Background(), id, source, isReadLock, Options{
		Timeout: drwMutexInfinite,
// GetRLock tries to get a read lock on dm before the timeout elapses.
// If one or more read locks are already in use, it will grant another lock.
// Otherwise the calling go routine blocks until either the mutex becomes
// available and return success or more time has passed than the timeout
// value and return false.
func (dm *DRWMutex) GetRLock(ctx context.Context, id, source string, opts Options) (locked bool) {
	isReadLock := true
	return dm.lockBlocking(ctx, id, source, isReadLock, opts)
	lockRetryInterval = 1 * time.Second
// lockBlocking will try to acquire either a read or a write lock
// The function will loop using a built-in timing randomized back-off
// algorithm until either the lock is acquired successfully or more
// time has elapsed than the timeout value.
func (dm *DRWMutex) lockBlocking(ctx context.Context, id, source string, isReadLock bool, opts Options) (locked bool) {
	restClnts, _ := dm.clnt.GetLockers()
	r := rand.New(rand.NewSource(time.Now().UnixNano()))
	// Create lock array to capture the successful lockers
	locks := make([]string, len(restClnts))
	log("lockBlocking %s/%s for %#v: lockType readLock(%t), additional opts: %#v\n", id, source, dm.Names, isReadLock, opts)
	// Add total timeout
	ctx, cancel := context.WithTimeout(ctx, opts.Timeout)
	defer cancel()
	// Tolerance is not set, defaults to half of the locker clients.
	tolerance := len(restClnts) / 2
	// Quorum is effectively = total clients subtracted with tolerance limit
	quorum := len(restClnts) - tolerance
	if !isReadLock {
		// In situations for write locks, as a special case
		// to avoid split brains we make sure to acquire
		// quorum + 1 when tolerance is exactly half of the
		// total locker clients.
		if quorum == tolerance {
			quorum++
	tolerance = len(restClnts) - quorum
		case <-ctx.Done():
			return false
			// Try to acquire the lock.
			if locked = lock(ctx, dm.clnt, &locks, id, source, isReadLock, tolerance, quorum, dm.Names...); locked {
				dm.m.Lock()
				// If success, copy array to object
				if isReadLock {
					// Append new array of strings at the end
					dm.readersLocks = append(dm.readersLocks, make([]string, len(restClnts)))
					// and copy stack array into last spot
					copy(dm.readersLocks[len(dm.readersLocks)-1], locks[:])
				} else {
					copy(dm.writeLocks, locks[:])
				dm.m.Unlock()
				log("lockBlocking %s/%s for %#v: granted\n", id, source, dm.Names)
				return locked
			time.Sleep(time.Duration(r.Float64() * float64(lockRetryInterval)))
// lock tries to acquire the distributed lock, returning true or false.
func lock(ctx context.Context, ds *Dsync, locks *[]string, id, source string, isReadLock bool, tolerance, quorum int, lockNames ...string) bool {
	for i := range *locks {
		(*locks)[i] = ""
	restClnts, owner := ds.GetLockers()
	// Create buffered channel of size equal to total number of nodes.
	ch := make(chan Granted, len(restClnts))
	var wg sync.WaitGroup
	// Combined timeout for the lock attempt.
	ctx, cancel := context.WithTimeout(ctx, DRWMutexAcquireTimeout)
	defer cancel()
	for index, c := range restClnts {
		wg.Add(1)
		// broadcast lock request to all nodes
		go func(index int, isReadLock bool, c NetLocker) {
			defer wg.Done()
			g := Granted{index: index}
			if c == nil {
				log("dsync: nil locker\n")
				ch <- g
			args := LockArgs{
				Owner:     owner,
				UID:       id,
				Resources: lockNames,
				Source:    source,
				Quorum:    quorum,
			var locked bool
			var err error
			if isReadLock {
				if locked, err = c.RLock(context.Background(), args); err != nil {
					log("dsync: Unable to call RLock failed with %s for %#v at %s\n", err, args, c)
			} else {
				if locked, err = c.Lock(context.Background(), args); err != nil {
					log("dsync: Unable to call Lock failed with %s for %#v at %s\n", err, args, c)
			if locked {
				g.lockUID = args.UID
		}(index, isReadLock, c)
	// Wait until we have either
	// a) received all lock responses
	// b) received too many 'non-'locks for quorum to be still possible
	// c) timed out
	i, locksFailed := 0, 0
	done := false
	for ; i < len(restClnts); i++ { // Loop until we acquired all locks
		case grant := <-ch:
			if grant.isLocked() {
				// Mark that this node has acquired the lock
				(*locks)[grant.index] = grant.lockUID
			} else {
				locksFailed++
				if locksFailed > tolerance {
					// We know that we are not going to get the lock anymore,
					// so exit out and release any locks that did get acquired
					done = true
		case <-ctx.Done():
			// Capture timedout locks as failed or took too long
			locksFailed++
			if locksFailed > tolerance {
				// We know that we are not going to get the lock anymore,
				// so exit out and release any locks that did get acquired
				done = true
		if done {
	quorumLocked := checkQuorumLocked(locks, quorum) && locksFailed <= tolerance
	if !quorumLocked {
		log("Releasing all acquired locks now abandoned after quorum was not met\n")
		if !releaseAll(ds, tolerance, owner, locks, isReadLock, restClnts, lockNames...) {
			log("Unable to release acquired locks, stale locks might be present\n")
	// We may have some unused results in ch, release them async.
	go func() {
		wg.Wait()
		close(ch)
		for grantToBeReleased := range ch {
			if grantToBeReleased.isLocked() {
				// release abandoned lock
				log("Releasing abandoned lock\n")
				sendRelease(ds, restClnts[grantToBeReleased.index],
					owner, grantToBeReleased.lockUID, isReadLock, lockNames...)
	return quorumLocked
// checkFailedUnlocks determines whether we have sufficiently unlocked all
// resources to ensure no deadlocks for future callers
func checkFailedUnlocks(locks []string, tolerance int) bool {
	unlocksFailed := 0
	for lockID := range locks {
		if isLocked(locks[lockID]) {
			unlocksFailed++
	// Unlock failures are higher than tolerance limit
	// for this instance of unlocker, we should let the
	// caller know that lock is not successfully released
	if len(locks)-tolerance == tolerance {
		// Incase of split brain scenarios where
		// tolerance is exactly half of the len(*locks)
		// then we need to make sure we have unlocked
		// upto tolerance+1 - especially for RUnlock
		// to ensure that we don't end up with active
		// read locks on the resource after unlocking
		// only half of the lockers.
		return unlocksFailed >= tolerance
	return unlocksFailed > tolerance
// checkQuorumLocked determines whether we have locked the required quorum of underlying locks or not
func checkQuorumLocked(locks *[]string, quorum int) bool {
	count := 0
	for _, uid := range *locks {
		if isLocked(uid) {
	return count >= quorum
// releaseAll releases all locks that are marked as locked
func releaseAll(ds *Dsync, tolerance int, owner string, locks *[]string, isReadLock bool, restClnts []NetLocker, lockNames ...string) bool {
	var wg sync.WaitGroup
	for lockID := range restClnts {
		wg.Add(1)
		go func(lockID int) {
			defer wg.Done()
			if isLocked((*locks)[lockID]) {
				if sendRelease(ds, restClnts[lockID], owner, (*locks)[lockID], isReadLock, lockNames...) {
					(*locks)[lockID] = ""
		}(lockID)
	// Return true if releaseAll was successful, otherwise we return 'false'
	// to indicate we haven't sufficiently unlocked lockers to avoid deadlocks.
	// Caller may use this as an indication to call again.
	return !checkFailedUnlocks(*locks, tolerance)
// Unlock unlocks the write lock.
// It is a run-time error if dm is not locked on entry to Unlock.
func (dm *DRWMutex) Unlock() {
	restClnts, owner := dm.clnt.GetLockers()
	// create temp array on stack
	locks := make([]string, len(restClnts))
		dm.m.Lock()
		defer dm.m.Unlock()
		// Check if minimally a single bool is set in the writeLocks array
		lockFound := false
		for _, uid := range dm.writeLocks {
			if isLocked(uid) {
				lockFound = true
		if !lockFound {
			panic("Trying to Unlock() while no Lock() is active")
		// Copy write locks to stack ar
7395
ray
		copy(locks, dm.writeLocks[:])
	// Tolerance is not set, defaults to half of the locker clients.
	tolerance := len(restClnts) / 2
	isReadLock := false
	r := rand.New(rand.NewSource(time.Now().UnixNano()))
	for !releaseAll(dm.clnt, tolerance, owner, &locks, isReadLock, restClnts, dm.Names...) {
		time.Sleep(time.Duration(r.Float64() * float64(lockRetryInterval)))
// RUnlock releases a read lock held on dm.
// It is a run-time error if dm is not locked on entry to RUnlock.
func (dm *DRWMutex) RUnlock() {
	// create temp array on stack
	restClnts, owner := dm.clnt.GetLockers()
	locks := make([]string, len(restClnts))
		dm.m.Lock()
		defer dm.m.Unlock()
		if len(dm.readersLocks) == 0 {
			panic("Trying to RUnlock() while no RLock() is active")
		// Copy out first element to release it first (FIFO)
		copy(locks, dm.readersLocks[0][:])
		// Drop first element from array
		dm.readersLocks = dm.readersLocks[1:]
	// Tolerance is not set, defaults to half of the locker clients.
	tolerance := len(restClnts) / 2
	isReadLock := true
	r := rand.New(rand.NewSource(time.Now().UnixNano()))
	for !releaseAll(dm.clnt, tolerance, owner, &locks, isReadLock, restClnts, dm.Names...) {
		time.Sleep(time.Duration(r.Float64() * float64(lockRetryInterval)))
// sendRelease sends a release message to a node that previously granted a lock
func sendRelease(ds *Dsync, c NetLocker, owner string, uid string, isReadLock bool, names ...string) bool {
	if c == nil {
		log("Unable to call RUnlock failed with %s\n", errors.New("netLocker is offline"))
		return false
	args := LockArgs{
		Owner:     owner,
		UID:       uid,
		Resources: names,
	if isReadLock {
		if _, err := c.RUnlock(args); err != nil {
			log("dsync: Unable to call RUnlock failed with %s for %#v at %s\n", err, args, c)
			return false
		if _, err := c.Unlock(args); err != nil {
			log("dsync: Unable to call Unlock failed with %s for %#v at %s\n", err, args, c)
			return false
	return true
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