github.com/hlts2/go@v0.0.0-20170904000733-812b34efaed8/src/sync/atomic/value.go (about)

     1  // Copyright 2014 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 atomic
     6  
     7  import (
     8  	"unsafe"
     9  )
    10  
    11  // A Value provides an atomic load and store of a consistently typed value.
    12  // The zero value for a Value returns nil from Load.
    13  // Once Store has been called, a Value must not be copied.
    14  //
    15  // A Value must not be copied after first use.
    16  type Value struct {
    17  	noCopy noCopy
    18  
    19  	v interface{}
    20  }
    21  
    22  // ifaceWords is interface{} internal representation.
    23  type ifaceWords struct {
    24  	typ  unsafe.Pointer
    25  	data unsafe.Pointer
    26  }
    27  
    28  // Load returns the value set by the most recent Store.
    29  // It returns nil if there has been no call to Store for this Value.
    30  func (v *Value) Load() (x interface{}) {
    31  	vp := (*ifaceWords)(unsafe.Pointer(v))
    32  	typ := LoadPointer(&vp.typ)
    33  	if typ == nil || uintptr(typ) == ^uintptr(0) {
    34  		// First store not yet completed.
    35  		return nil
    36  	}
    37  	data := LoadPointer(&vp.data)
    38  	xp := (*ifaceWords)(unsafe.Pointer(&x))
    39  	xp.typ = typ
    40  	xp.data = data
    41  	return
    42  }
    43  
    44  // Store sets the value of the Value to x.
    45  // All calls to Store for a given Value must use values of the same concrete type.
    46  // Store of an inconsistent type panics, as does Store(nil).
    47  func (v *Value) Store(x interface{}) {
    48  	if x == nil {
    49  		panic("sync/atomic: store of nil value into Value")
    50  	}
    51  	vp := (*ifaceWords)(unsafe.Pointer(v))
    52  	xp := (*ifaceWords)(unsafe.Pointer(&x))
    53  	for {
    54  		typ := LoadPointer(&vp.typ)
    55  		if typ == nil {
    56  			// Attempt to start first store.
    57  			// Disable preemption so that other goroutines can use
    58  			// active spin wait to wait for completion; and so that
    59  			// GC does not see the fake type accidentally.
    60  			runtime_procPin()
    61  			if !CompareAndSwapPointer(&vp.typ, nil, unsafe.Pointer(^uintptr(0))) {
    62  				runtime_procUnpin()
    63  				continue
    64  			}
    65  			// Complete first store.
    66  			StorePointer(&vp.data, xp.data)
    67  			StorePointer(&vp.typ, xp.typ)
    68  			runtime_procUnpin()
    69  			return
    70  		}
    71  		if uintptr(typ) == ^uintptr(0) {
    72  			// First store in progress. Wait.
    73  			// Since we disable preemption around the first store,
    74  			// we can wait with active spinning.
    75  			continue
    76  		}
    77  		// First store completed. Check type and overwrite data.
    78  		if typ != xp.typ {
    79  			panic("sync/atomic: store of inconsistently typed value into Value")
    80  		}
    81  		StorePointer(&vp.data, xp.data)
    82  		return
    83  	}
    84  }
    85  
    86  // Disable/enable preemption, implemented in runtime.
    87  func runtime_procPin()
    88  func runtime_procUnpin()
    89  
    90  // noCopy may be embedded into structs which must not be copied
    91  // after the first use.
    92  //
    93  // See https://github.com/golang/go/issues/8005#issuecomment-190753527
    94  // for details.
    95  type noCopy struct{}
    96  
    97  // Lock is a no-op used by -copylocks checker from `go vet`.
    98  func (*noCopy) Lock() {}