| // Copyright 2009 The Go Authors. All rights reserved. |
| // Use of this source code is governed by a BSD-style |
| // license that can be found in the LICENSE file. |
| |
| // The sync package provides basic synchronization primitives |
| // such as mutual exclusion locks. Other than the Once type, |
| // most are intended for use by low-level library routines. |
| // Higher-level synchronization is better done via channels |
| // and communication. |
| package sync |
| |
| import "runtime" |
| |
| func cas(val *uint32, old, new uint32) bool |
| |
| // A Mutex is a mutual exclusion lock. |
| // Mutexes can be created as part of other structures; |
| // the zero value for a Mutex is an unlocked mutex. |
| type Mutex struct { |
| key uint32 |
| sema uint32 |
| } |
| |
| // Add delta to *val, and return the new *val in a thread-safe way. If multiple |
| // goroutines call xadd on the same val concurrently, the changes will be |
| // serialized, and all the deltas will be added in an undefined order. |
| func xadd(val *uint32, delta int32) (new uint32) { |
| for { |
| v := *val |
| nv := v + uint32(delta) |
| if cas(val, v, nv) { |
| return nv |
| } |
| } |
| panic("unreached") |
| } |
| |
| // Lock locks m. |
| // If the lock is already in use, the calling goroutine |
| // blocks until the mutex is available. |
| func (m *Mutex) Lock() { |
| if xadd(&m.key, 1) == 1 { |
| // changed from 0 to 1; we hold lock |
| return |
| } |
| runtime.Semacquire(&m.sema) |
| } |
| |
| // Unlock unlocks m. |
| // It is a run-time error if m is not locked on entry to Unlock. |
| // |
| // A locked Mutex is not associated with a particular goroutine. |
| // It is allowed for one goroutine to lock a Mutex and then |
| // arrange for another goroutine to unlock it. |
| func (m *Mutex) Unlock() { |
| if xadd(&m.key, -1) == 0 { |
| // changed from 1 to 0; no contention |
| return |
| } |
| runtime.Semrelease(&m.sema) |
| } |