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Copy pathlrwmutex.go
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186 lines (161 loc) · 4.78 KB
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// Copyright (c) 2015-2021 MinIO, Inc.
//
// This file is part of MinIO Object Storage stack
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU Affero General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Affero General Public License for more details.
//
// You should have received a copy of the GNU Affero General Public License
// along with this program. If not, see <http://www.gnu.org/licenses/>.
package lsync
import (
"context"
"math"
"math/rand"
"sync"
"time"
)
// A LRWMutex is a mutual exclusion lock with timeouts.
type LRWMutex struct {
id string
source string
isWriteLock bool
ref int
mu sync.Mutex // Mutex to prevent multiple simultaneous locks
}
// NewLRWMutex - initializes a new lsync RW mutex.
func NewLRWMutex() *LRWMutex {
return &LRWMutex{}
}
// Lock holds a write lock on lm.
//
// If the lock is already in use, the calling go rou
8000
tine
// blocks until the mutex is available.
func (lm *LRWMutex) Lock() {
const isWriteLock = true
lm.lockLoop(context.Background(), lm.id, lm.source, math.MaxInt64, isWriteLock)
}
// GetLock tries to get a write lock on lm before the timeout occurs.
func (lm *LRWMutex) GetLock(ctx context.Context, id string, source string, timeout time.Duration) (locked bool) {
const isWriteLock = true
return lm.lockLoop(ctx, id, source, timeout, isWriteLock)
}
// RLock holds a read lock on lm.
//
// If one or more read lock are already in use, it will grant another lock.
// Otherwise the calling go routine blocks until the mutex is available.
func (lm *LRWMutex) RLock() {
const isWriteLock = false
lm.lockLoop(context.Background(), lm.id, lm.source, 1<<63-1, isWriteLock)
}
// GetRLock tries to get a read lock on lm before the timeout occurs.
func (lm *LRWMutex) GetRLock(ctx context.Context, id string, source string, timeout time.Duration) (locked bool) {
const isWriteLock = false
return lm.lockLoop(ctx, id, source, timeout, isWriteLock)
}
func (lm *LRWMutex) lock(id, source string, isWriteLock bool) (locked bool) {
lm.mu.Lock()
defer lm.mu.Unlock()
lm.id = id
lm.source = source
if isWriteLock {
if lm.ref == 0 && !lm.isWriteLock {
lm.ref = 1
lm.isWriteLock = true
locked = true
}
} else {
if !lm.isWriteLock {
lm.ref++
locked = true
}
}
return locked
}
const (
lockRetryInterval = 50 * time.Millisecond
)
// lockLoop will acquire either a read or a write lock
//
// The call will block until the lock is granted using a built-in
// timing randomized back-off algorithm to try again until successful
func (lm *LRWMutex) lockLoop(ctx context.Context, id, source string, timeout time.Duration, isWriteLock bool) (locked bool) {
r := rand.New(rand.NewSource(time.Now().UnixNano()))
retryCtx, cancel := context.WithTimeout(ctx, timeout)
defer cancel()
for {
select {
case <-retryCtx.Done():
// Caller context canceled or we timedout,
// return false anyways for both situations.
return false
default:
if lm.lock(id, source, isWriteLock) {
return true
}
time.Sleep(time.Duration(r.Float64() * float64(lockRetryInterval)))
}
}
}
// Unlock unlocks the write lock.
//
// It is a run-time error if lm is not locked on entry to Unlock.
func (lm *LRWMutex) Unlock() {
isWriteLock := true
success := lm.unlock(isWriteLock)
if !success {
panic("Trying to Unlock() while no Lock() is active")
}
}
// RUnlock releases a read lock held on lm.
//
// It is a run-time error if lm is not locked on entry to RUnlock.
func (lm *LRWMutex) RUnlock() {
isWriteLock := false
success := lm.unlock(isWriteLock)
if !success {
panic("Trying to RUnlock() while no RLock() is active")
}
}
func (lm *LRWMutex) unlock(isWriteLock bool) (unlocked bool) {
lm.mu.Lock()
defer lm.mu.Unlock()
// Try to release lock.
if isWriteLock {
if lm.isWriteLock && lm.ref == 1 {
lm.ref = 0
lm.isWriteLock = false
unlocked = true
}
} else {
if !lm.isWriteLock {
if lm.ref > 0 {
lm.ref--
unlocked = true
}
}
}
return unlocked
}
// ForceUnlock will forcefully clear a write or read lock.
func (lm *LRWMutex) ForceUnlock() {
lm.mu.Lock()
defer lm.mu.Unlock()
lm.ref = 0
lm.isWriteLock = false
}
// DRLocker returns a sync.Locker interface that implements
// the Lock and Unlock methods by calling drw.RLock and drw.RUnlock.
func (lm *LRWMutex) DRLocker() sync.Locker {
return (*drlocker)(lm)
}
type drlocker LRWMutex
func (dr *drlocker) Lock() { (*LRWMutex)(dr).RLock() }
func (dr *drlocker) Unlock() { (*LRWMutex)(dr).RUnlock() }