Source file src/sync/mutex.go

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // Package sync provides basic synchronization primitives such as mutual
     6  // exclusion locks. Other than the Once and WaitGroup types, most are intended
     7  // for use by low-level library routines. Higher-level synchronization is
     8  // better done via channels and communication.
     9  //
    10  // Values containing the types defined in this package should not be copied.
    11  package sync
    12  
    13  import (
    14  	"internal/race"
    15  	"sync/atomic"
    16  	"unsafe"
    17  )
    18  
    19  // Provided by runtime via linkname.
    20  func throw(string)
    21  func fatal(string)
    22  
    23  // A Mutex is a mutual exclusion lock.
    24  // The zero value for a Mutex is an unlocked mutex.
    25  //
    26  // A Mutex must not be copied after first use.
    27  //
    28  // In the terminology of the Go memory model,
    29  // the n'th call to Unlock “synchronizes before” the m'th call to Lock
    30  // for any n < m.
    31  // A successful call to TryLock is equivalent to a call to Lock.
    32  // A failed call to TryLock does not establish any “synchronizes before”
    33  // relation at all.
    34  type Mutex struct {
    35  	state int32
    36  	sema  uint32
    37  }
    38  
    39  // A Locker represents an object that can be locked and unlocked.
    40  type Locker interface {
    41  	Lock()
    42  	Unlock()
    43  }
    44  
    45  const (
    46  	mutexLocked = 1 << iota // mutex is locked
    47  	mutexWoken
    48  	mutexStarving
    49  	mutexWaiterShift = iota
    50  
    51  	// Mutex fairness.
    52  	//
    53  	// Mutex can be in 2 modes of operations: normal and starvation.
    54  	// In normal mode waiters are queued in FIFO order, but a woken up waiter
    55  	// does not own the mutex and competes with new arriving goroutines over
    56  	// the ownership. New arriving goroutines have an advantage -- they are
    57  	// already running on CPU and there can be lots of them, so a woken up
    58  	// waiter has good chances of losing. In such case it is queued at front
    59  	// of the wait queue. If a waiter fails to acquire the mutex for more than 1ms,
    60  	// it switches mutex to the starvation mode.
    61  	//
    62  	// In starvation mode ownership of the mutex is directly handed off from
    63  	// the unlocking goroutine to the waiter at the front of the queue.
    64  	// New arriving goroutines don't try to acquire the mutex even if it appears
    65  	// to be unlocked, and don't try to spin. Instead they queue themselves at
    66  	// the tail of the wait queue.
    67  	//
    68  	// If a waiter receives ownership of the mutex and sees that either
    69  	// (1) it is the last waiter in the queue, or (2) it waited for less than 1 ms,
    70  	// it switches mutex back to normal operation mode.
    71  	//
    72  	// Normal mode has considerably better performance as a goroutine can acquire
    73  	// a mutex several times in a row even if there are blocked waiters.
    74  	// Starvation mode is important to prevent pathological cases of tail latency.
    75  	starvationThresholdNs = 1e6
    76  )
    77  
    78  // Lock locks m.
    79  // If the lock is already in use, the calling goroutine
    80  // blocks until the mutex is available.
    81  func (m *Mutex) Lock() {
    82  	// Fast path: grab unlocked mutex.
    83  	if atomic.CompareAndSwapInt32(&m.state, 0, mutexLocked) {
    84  		if race.Enabled {
    85  			race.Acquire(unsafe.Pointer(m))
    86  		}
    87  		return
    88  	}
    89  	// Slow path (outlined so that the fast path can be inlined)
    90  	m.lockSlow()
    91  }
    92  
    93  // TryLock tries to lock m and reports whether it succeeded.
    94  //
    95  // Note that while correct uses of TryLock do exist, they are rare,
    96  // and use of TryLock is often a sign of a deeper problem
    97  // in a particular use of mutexes.
    98  func (m *Mutex) TryLock() bool {
    99  	old := m.state
   100  	if old&(mutexLocked|mutexStarving) != 0 {
   101  		return false
   102  	}
   103  
   104  	// There may be a goroutine waiting for the mutex, but we are
   105  	// running now and can try to grab the mutex before that
   106  	// goroutine wakes up.
   107  	if !atomic.CompareAndSwapInt32(&m.state, old, old|mutexLocked) {
   108  		return false
   109  	}
   110  
   111  	if race.Enabled {
   112  		race.Acquire(unsafe.Pointer(m))
   113  	}
   114  	return true
   115  }
   116  
   117  func (m *Mutex) lockSlow() {
   118  	var waitStartTime int64
   119  	starving := false
   120  	awoke := false
   121  	iter := 0
   122  	old := m.state
   123  	for {
   124  		// Don't spin in starvation mode, ownership is handed off to waiters
   125  		// so we won't be able to acquire the mutex anyway.
   126  		if old&(mutexLocked|mutexStarving) == mutexLocked && runtime_canSpin(iter) {
   127  			// Active spinning makes sense.
   128  			// Try to set mutexWoken flag to inform Unlock
   129  			// to not wake other blocked goroutines.
   130  			if !awoke && old&mutexWoken == 0 && old>>mutexWaiterShift != 0 &&
   131  				atomic.CompareAndSwapInt32(&m.state, old, old|mutexWoken) {
   132  				awoke = true
   133  			}
   134  			runtime_doSpin()
   135  			iter++
   136  			old = m.state
   137  			continue
   138  		}
   139  		new := old
   140  		// Don't try to acquire starving mutex, new arriving goroutines must queue.
   141  		if old&mutexStarving == 0 {
   142  			new |= mutexLocked
   143  		}
   144  		if old&(mutexLocked|mutexStarving) != 0 {
   145  			new += 1 << mutexWaiterShift
   146  		}
   147  		// The current goroutine switches mutex to starvation mode.
   148  		// But if the mutex is currently unlocked, don't do the switch.
   149  		// Unlock expects that starving mutex has waiters, which will not
   150  		// be true in this case.
   151  		if starving && old&mutexLocked != 0 {
   152  			new |= mutexStarving
   153  		}
   154  		if awoke {
   155  			// The goroutine has been woken from sleep,
   156  			// so we need to reset the flag in either case.
   157  			if new&mutexWoken == 0 {
   158  				throw("sync: inconsistent mutex state")
   159  			}
   160  			new &^= mutexWoken
   161  		}
   162  		if atomic.CompareAndSwapInt32(&m.state, old, new) {
   163  			if old&(mutexLocked|mutexStarving) == 0 {
   164  				break // locked the mutex with CAS
   165  			}
   166  			// If we were already waiting before, queue at the front of the queue.
   167  			queueLifo := waitStartTime != 0
   168  			if waitStartTime == 0 {
   169  				waitStartTime = runtime_nanotime()
   170  			}
   171  			runtime_SemacquireMutex(&m.sema, queueLifo, 1)
   172  			starving = starving || runtime_nanotime()-waitStartTime > starvationThresholdNs
   173  			old = m.state
   174  			if old&mutexStarving != 0 {
   175  				// If this goroutine was woken and mutex is in starvation mode,
   176  				// ownership was handed off to us but mutex is in somewhat
   177  				// inconsistent state: mutexLocked is not set and we are still
   178  				// accounted as waiter. Fix that.
   179  				if old&(mutexLocked|mutexWoken) != 0 || old>>mutexWaiterShift == 0 {
   180  					throw("sync: inconsistent mutex state")
   181  				}
   182  				delta := int32(mutexLocked - 1<<mutexWaiterShift)
   183  				if !starving || old>>mutexWaiterShift == 1 {
   184  					// Exit starvation mode.
   185  					// Critical to do it here and consider wait time.
   186  					// Starvation mode is so inefficient, that two goroutines
   187  					// can go lock-step infinitely once they switch mutex
   188  					// to starvation mode.
   189  					delta -= mutexStarving
   190  				}
   191  				atomic.AddInt32(&m.state, delta)
   192  				break
   193  			}
   194  			awoke = true
   195  			iter = 0
   196  		} else {
   197  			old = m.state
   198  		}
   199  	}
   200  
   201  	if race.Enabled {
   202  		race.Acquire(unsafe.Pointer(m))
   203  	}
   204  }
   205  
   206  // Unlock unlocks m.
   207  // It is a run-time error if m is not locked on entry to Unlock.
   208  //
   209  // A locked Mutex is not associated with a particular goroutine.
   210  // It is allowed for one goroutine to lock a Mutex and then
   211  // arrange for another goroutine to unlock it.
   212  func (m *Mutex) Unlock() {
   213  	if race.Enabled {
   214  		_ = m.state
   215  		race.Release(unsafe.Pointer(m))
   216  	}
   217  
   218  	// Fast path: drop lock bit.
   219  	new := atomic.AddInt32(&m.state, -mutexLocked)
   220  	if new != 0 {
   221  		// Outlined slow path to allow inlining the fast path.
   222  		// To hide unlockSlow during tracing we skip one extra frame when tracing GoUnblock.
   223  		m.unlockSlow(new)
   224  	}
   225  }
   226  
   227  func (m *Mutex) unlockSlow(new int32) {
   228  	if (new+mutexLocked)&mutexLocked == 0 {
   229  		fatal("sync: unlock of unlocked mutex")
   230  	}
   231  	if new&mutexStarving == 0 {
   232  		old := new
   233  		for {
   234  			// If there are no waiters or a goroutine has already
   235  			// been woken or grabbed the lock, no need to wake anyone.
   236  			// In starvation mode ownership is directly handed off from unlocking
   237  			// goroutine to the next waiter. We are not part of this chain,
   238  			// since we did not observe mutexStarving when we unlocked the mutex above.
   239  			// So get off the way.
   240  			if old>>mutexWaiterShift == 0 || old&(mutexLocked|mutexWoken|mutexStarving) != 0 {
   241  				return
   242  			}
   243  			// Grab the right to wake someone.
   244  			new = (old - 1<<mutexWaiterShift) | mutexWoken
   245  			if atomic.CompareAndSwapInt32(&m.state, old, new) {
   246  				runtime_Semrelease(&m.sema, false, 1)
   247  				return
   248  			}
   249  			old = m.state
   250  		}
   251  	} else {
   252  		// Starving mode: handoff mutex ownership to the next waiter, and yield
   253  		// our time slice so that the next waiter can start to run immediately.
   254  		// Note: mutexLocked is not set, the waiter will set it after wakeup.
   255  		// But mutex is still considered locked if mutexStarving is set,
   256  		// so new coming goroutines won't acquire it.
   257  		runtime_Semrelease(&m.sema, true, 1)
   258  	}
   259  }
   260  

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