Source file src/sync/poolqueue.go

Documentation: sync

     1  // Copyright 2019 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
     6  
     7  import (
     8  	"sync/atomic"
     9  	"unsafe"
    10  )
    11  
    12  // poolDequeue is a lock-free fixed-size single-producer,
    13  // multi-consumer queue. The single producer can both push and pop
    14  // from the head, and consumers can pop from the tail.
    15  //
    16  // It has the added feature that it nils out unused slots to avoid
    17  // unnecessary retention of objects. This is important for sync.Pool,
    18  // but not typically a property considered in the literature.
    19  type poolDequeue struct {
    20  	// headTail packs together a 32-bit head index and a 32-bit
    21  	// tail index. Both are indexes into vals modulo len(vals)-1.
    22  	//
    23  	// tail = index of oldest data in queue
    24  	// head = index of next slot to fill
    25  	//
    26  	// Slots in the range [tail, head) are owned by consumers.
    27  	// A consumer continues to own a slot outside this range until
    28  	// it nils the slot, at which point ownership passes to the
    29  	// producer.
    30  	//
    31  	// The head index is stored in the most-significant bits so
    32  	// that we can atomically add to it and the overflow is
    33  	// harmless.
    34  	headTail uint64
    35  
    36  	// vals is a ring buffer of interface{} values stored in this
    37  	// dequeue. The size of this must be a power of 2.
    38  	//
    39  	// vals[i].typ is nil if the slot is empty and non-nil
    40  	// otherwise. A slot is still in use until *both* the tail
    41  	// index has moved beyond it and typ has been set to nil. This
    42  	// is set to nil atomically by the consumer and read
    43  	// atomically by the producer.
    44  	vals []eface
    45  }
    46  
    47  type eface struct {
    48  	typ, val unsafe.Pointer
    49  }
    50  
    51  const dequeueBits = 32
    52  
    53  // dequeueLimit is the maximum size of a poolDequeue.
    54  //
    55  // This must be at most (1<<dequeueBits)/2 because detecting fullness
    56  // depends on wrapping around the ring buffer without wrapping around
    57  // the index. We divide by 4 so this fits in an int on 32-bit.
    58  const dequeueLimit = (1 << dequeueBits) / 4
    59  
    60  // dequeueNil is used in poolDeqeue to represent interface{}(nil).
    61  // Since we use nil to represent empty slots, we need a sentinel value
    62  // to represent nil.
    63  type dequeueNil *struct{}
    64  
    65  func (d *poolDequeue) unpack(ptrs uint64) (head, tail uint32) {
    66  	const mask = 1<<dequeueBits - 1
    67  	head = uint32((ptrs >> dequeueBits) & mask)
    68  	tail = uint32(ptrs & mask)
    69  	return
    70  }
    71  
    72  func (d *poolDequeue) pack(head, tail uint32) uint64 {
    73  	const mask = 1<<dequeueBits - 1
    74  	return (uint64(head) << dequeueBits) |
    75  		uint64(tail&mask)
    76  }
    77  
    78  // pushHead adds val at the head of the queue. It returns false if the
    79  // queue is full. It must only be called by a single producer.
    80  func (d *poolDequeue) pushHead(val interface{}) bool {
    81  	ptrs := atomic.LoadUint64(&d.headTail)
    82  	head, tail := d.unpack(ptrs)
    83  	if (tail+uint32(len(d.vals)))&(1<<dequeueBits-1) == head {
    84  		// Queue is full.
    85  		return false
    86  	}
    87  	slot := &d.vals[head&uint32(len(d.vals)-1)]
    88  
    89  	// Check if the head slot has been released by popTail.
    90  	typ := atomic.LoadPointer(&slot.typ)
    91  	if typ != nil {
    92  		// Another goroutine is still cleaning up the tail, so
    93  		// the queue is actually still full.
    94  		return false
    95  	}
    96  
    97  	// The head slot is free, so we own it.
    98  	if val == nil {
    99  		val = dequeueNil(nil)
   100  	}
   101  	*(*interface{})(unsafe.Pointer(slot)) = val
   102  
   103  	// Increment head. This passes ownership of slot to popTail
   104  	// and acts as a store barrier for writing the slot.
   105  	atomic.AddUint64(&d.headTail, 1<<dequeueBits)
   106  	return true
   107  }
   108  
   109  // popHead removes and returns the element at the head of the queue.
   110  // It returns false if the queue is empty. It must only be called by a
   111  // single producer.
   112  func (d *poolDequeue) popHead() (interface{}, bool) {
   113  	var slot *eface
   114  	for {
   115  		ptrs := atomic.LoadUint64(&d.headTail)
   116  		head, tail := d.unpack(ptrs)
   117  		if tail == head {
   118  			// Queue is empty.
   119  			return nil, false
   120  		}
   121  
   122  		// Confirm tail and decrement head. We do this before
   123  		// reading the value to take back ownership of this
   124  		// slot.
   125  		head--
   126  		ptrs2 := d.pack(head, tail)
   127  		if atomic.CompareAndSwapUint64(&d.headTail, ptrs, ptrs2) {
   128  			// We successfully took back slot.
   129  			slot = &d.vals[head&uint32(len(d.vals)-1)]
   130  			break
   131  		}
   132  	}
   133  
   134  	val := *(*interface{})(unsafe.Pointer(slot))
   135  	if val == dequeueNil(nil) {
   136  		val = nil
   137  	}
   138  	// Zero the slot. Unlike popTail, this isn't racing with
   139  	// pushHead, so we don't need to be careful here.
   140  	*slot = eface{}
   141  	return val, true
   142  }
   143  
   144  // popTail removes and returns the element at the tail of the queue.
   145  // It returns false if the queue is empty. It may be called by any
   146  // number of consumers.
   147  func (d *poolDequeue) popTail() (interface{}, bool) {
   148  	var slot *eface
   149  	for {
   150  		ptrs := atomic.LoadUint64(&d.headTail)
   151  		head, tail := d.unpack(ptrs)
   152  		if tail == head {
   153  			// Queue is empty.
   154  			return nil, false
   155  		}
   156  
   157  		// Confirm head and tail (for our speculative check
   158  		// above) and increment tail. If this succeeds, then
   159  		// we own the slot at tail.
   160  		ptrs2 := d.pack(head, tail+1)
   161  		if atomic.CompareAndSwapUint64(&d.headTail, ptrs, ptrs2) {
   162  			// Success.
   163  			slot = &d.vals[tail&uint32(len(d.vals)-1)]
   164  			break
   165  		}
   166  	}
   167  
   168  	// We now own slot.
   169  	val := *(*interface{})(unsafe.Pointer(slot))
   170  	if val == dequeueNil(nil) {
   171  		val = nil
   172  	}
   173  
   174  	// Tell pushHead that we're done with this slot. Zeroing the
   175  	// slot is also important so we don't leave behind references
   176  	// that could keep this object live longer than necessary.
   177  	//
   178  	// We write to val first and then publish that we're done with
   179  	// this slot by atomically writing to typ.
   180  	slot.val = nil
   181  	atomic.StorePointer(&slot.typ, nil)
   182  	// At this point pushHead owns the slot.
   183  
   184  	return val, true
   185  }
   186  
   187  // poolChain is a dynamically-sized version of poolDequeue.
   188  //
   189  // This is implemented as a doubly-linked list queue of poolDequeues
   190  // where each dequeue is double the size of the previous one. Once a
   191  // dequeue fills up, this allocates a new one and only ever pushes to
   192  // the latest dequeue. Pops happen from the other end of the list and
   193  // once a dequeue is exhausted, it gets removed from the list.
   194  type poolChain struct {
   195  	// head is the poolDequeue to push to. This is only accessed
   196  	// by the producer, so doesn't need to be synchronized.
   197  	head *poolChainElt
   198  
   199  	// tail is the poolDequeue to popTail from. This is accessed
   200  	// by consumers, so reads and writes must be atomic.
   201  	tail *poolChainElt
   202  }
   203  
   204  type poolChainElt struct {
   205  	poolDequeue
   206  
   207  	// next and prev link to the adjacent poolChainElts in this
   208  	// poolChain.
   209  	//
   210  	// next is written atomically by the producer and read
   211  	// atomically by the consumer. It only transitions from nil to
   212  	// non-nil.
   213  	//
   214  	// prev is written atomically by the consumer and read
   215  	// atomically by the producer. It only transitions from
   216  	// non-nil to nil.
   217  	next, prev *poolChainElt
   218  }
   219  
   220  func storePoolChainElt(pp **poolChainElt, v *poolChainElt) {
   221  	atomic.StorePointer((*unsafe.Pointer)(unsafe.Pointer(pp)), unsafe.Pointer(v))
   222  }
   223  
   224  func loadPoolChainElt(pp **poolChainElt) *poolChainElt {
   225  	return (*poolChainElt)(atomic.LoadPointer((*unsafe.Pointer)(unsafe.Pointer(pp))))
   226  }
   227  
   228  func (c *poolChain) pushHead(val interface{}) {
   229  	d := c.head
   230  	if d == nil {
   231  		// Initialize the chain.
   232  		const initSize = 8 // Must be a power of 2
   233  		d = new(poolChainElt)
   234  		d.vals = make([]eface, initSize)
   235  		c.head = d
   236  		storePoolChainElt(&c.tail, d)
   237  	}
   238  
   239  	if d.pushHead(val) {
   240  		return
   241  	}
   242  
   243  	// The current dequeue is full. Allocate a new one of twice
   244  	// the size.
   245  	newSize := len(d.vals) * 2
   246  	if newSize >= dequeueLimit {
   247  		// Can't make it any bigger.
   248  		newSize = dequeueLimit
   249  	}
   250  
   251  	d2 := &poolChainElt{prev: d}
   252  	d2.vals = make([]eface, newSize)
   253  	c.head = d2
   254  	storePoolChainElt(&d.next, d2)
   255  	d2.pushHead(val)
   256  }
   257  
   258  func (c *poolChain) popHead() (interface{}, bool) {
   259  	d := c.head
   260  	for d != nil {
   261  		if val, ok := d.popHead(); ok {
   262  			return val, ok
   263  		}
   264  		// There may still be unconsumed elements in the
   265  		// previous dequeue, so try backing up.
   266  		d = loadPoolChainElt(&d.prev)
   267  	}
   268  	return nil, false
   269  }
   270  
   271  func (c *poolChain) popTail() (interface{}, bool) {
   272  	d := loadPoolChainElt(&c.tail)
   273  	if d == nil {
   274  		return nil, false
   275  	}
   276  
   277  	for {
   278  		// It's important that we load the next pointer
   279  		// *before* popping the tail. In general, d may be
   280  		// transiently empty, but if next is non-nil before
   281  		// the pop and the pop fails, then d is permanently
   282  		// empty, which is the only condition under which it's
   283  		// safe to drop d from the chain.
   284  		d2 := loadPoolChainElt(&d.next)
   285  
   286  		if val, ok := d.popTail(); ok {
   287  			return val, ok
   288  		}
   289  
   290  		if d2 == nil {
   291  			// This is the only dequeue. It's empty right
   292  			// now, but could be pushed to in the future.
   293  			return nil, false
   294  		}
   295  
   296  		// The tail of the chain has been drained, so move on
   297  		// to the next dequeue. Try to drop it from the chain
   298  		// so the next pop doesn't have to look at the empty
   299  		// dequeue again.
   300  		if atomic.CompareAndSwapPointer((*unsafe.Pointer)(unsafe.Pointer(&c.tail)), unsafe.Pointer(d), unsafe.Pointer(d2)) {
   301  			// We won the race. Clear the prev pointer so
   302  			// the garbage collector can collect the empty
   303  			// dequeue and so popHead doesn't back up
   304  			// further than necessary.
   305  			storePoolChainElt(&d2.prev, nil)
   306  		}
   307  		d = d2
   308  	}
   309  }
   310  

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