github.com/tidwall/go@v0.0.0-20170415222209-6694a6888b7d/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  // Values can be created as part of other data structures.
    13  // The zero value for a Value returns nil from Load.
    14  // Once Store has been called, a Value must not be copied.
    15  //
    16  // A Value must not be copied after first use.
    17  type Value struct {
    18  	noCopy noCopy
    19  
    20  	v interface{}
    21  }
    22  
    23  // ifaceWords is interface{} internal representation.
    24  type ifaceWords struct {
    25  	typ  unsafe.Pointer
    26  	data unsafe.Pointer
    27  }
    28  
    29  // Load returns the value set by the most recent Store.
    30  // It returns nil if there has been no call to Store for this Value.
    31  func (v *Value) Load() (x interface{}) {
    32  	vp := (*ifaceWords)(unsafe.Pointer(v))
    33  	typ := LoadPointer(&vp.typ)
    34  	if typ == nil || uintptr(typ) == ^uintptr(0) {
    35  		// First store not yet completed.
    36  		return nil
    37  	}
    38  	data := LoadPointer(&vp.data)
    39  	xp := (*ifaceWords)(unsafe.Pointer(&x))
    40  	xp.typ = typ
    41  	xp.data = data
    42  	return
    43  }
    44  
    45  // Store sets the value of the Value to x.
    46  // All calls to Store for a given Value must use values of the same concrete type.
    47  // Store of an inconsistent type panics, as does Store(nil).
    48  func (v *Value) Store(x interface{}) {
    49  	if x == nil {
    50  		panic("sync/atomic: store of nil value into Value")
    51  	}
    52  	vp := (*ifaceWords)(unsafe.Pointer(v))
    53  	xp := (*ifaceWords)(unsafe.Pointer(&x))
    54  	for {
    55  		typ := LoadPointer(&vp.typ)
    56  		if typ == nil {
    57  			// Attempt to start first store.
    58  			// Disable preemption so that other goroutines can use
    59  			// active spin wait to wait for completion; and so that
    60  			// GC does not see the fake type accidentally.
    61  			runtime_procPin()
    62  			if !CompareAndSwapPointer(&vp.typ, nil, unsafe.Pointer(^uintptr(0))) {
    63  				runtime_procUnpin()
    64  				continue
    65  			}
    66  			// Complete first store.
    67  			StorePointer(&vp.data, xp.data)
    68  			StorePointer(&vp.typ, xp.typ)
    69  			runtime_procUnpin()
    70  			return
    71  		}
    72  		if uintptr(typ) == ^uintptr(0) {
    73  			// First store in progress. Wait.
    74  			// Since we disable preemption around the first store,
    75  			// we can wait with active spinning.
    76  			continue
    77  		}
    78  		// First store completed. Check type and overwrite data.
    79  		if typ != xp.typ {
    80  			panic("sync/atomic: store of inconsistently typed value into Value")
    81  		}
    82  		StorePointer(&vp.data, xp.data)
    83  		return
    84  	}
    85  }
    86  
    87  // Disable/enable preemption, implemented in runtime.
    88  func runtime_procPin()
    89  func runtime_procUnpin()
    90  
    91  // noCopy may be embedded into structs which must not be copied
    92  // after the first use.
    93  //
    94  // See https://github.com/golang/go/issues/8005#issuecomment-190753527
    95  // for details.
    96  type noCopy struct{}
    97  
    98  // Lock is a no-op used by -copylocks checker from `go vet`.
    99  func (*noCopy) Lock() {}