Source file src/pkg/sync/atomic/value.go

     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		v interface{}
    18	}
    19	
    20	// ifaceWords is interface{} internal representation.
    21	type ifaceWords struct {
    22		typ  unsafe.Pointer
    23		data unsafe.Pointer
    24	}
    25	
    26	// Load returns the value set by the most recent Store.
    27	// It returns nil if there has been no call to Store for this Value.
    28	func (v *Value) Load() (x interface{}) {
    29		vp := (*ifaceWords)(unsafe.Pointer(v))
    30		typ := LoadPointer(&vp.typ)
    31		if typ == nil || uintptr(typ) == ^uintptr(0) {
    32			// First store not yet completed.
    33			return nil
    34		}
    35		data := LoadPointer(&vp.data)
    36		xp := (*ifaceWords)(unsafe.Pointer(&x))
    37		xp.typ = typ
    38		xp.data = data
    39		return
    40	}
    41	
    42	// Store sets the value of the Value to x.
    43	// All calls to Store for a given Value must use values of the same concrete type.
    44	// Store of an inconsistent type panics, as does Store(nil).
    45	func (v *Value) Store(x interface{}) {
    46		if x == nil {
    47			panic("sync/atomic: store of nil value into Value")
    48		}
    49		vp := (*ifaceWords)(unsafe.Pointer(v))
    50		xp := (*ifaceWords)(unsafe.Pointer(&x))
    51		for {
    52			typ := LoadPointer(&vp.typ)
    53			if typ == nil {
    54				// Attempt to start first store.
    55				// Disable preemption so that other goroutines can use
    56				// active spin wait to wait for completion; and so that
    57				// GC does not see the fake type accidentally.
    58				runtime_procPin()
    59				if !CompareAndSwapPointer(&vp.typ, nil, unsafe.Pointer(^uintptr(0))) {
    60					runtime_procUnpin()
    61					continue
    62				}
    63				// Complete first store.
    64				StorePointer(&vp.data, xp.data)
    65				StorePointer(&vp.typ, xp.typ)
    66				runtime_procUnpin()
    67				return
    68			}
    69			if uintptr(typ) == ^uintptr(0) {
    70				// First store in progress. Wait.
    71				// Since we disable preemption around the first store,
    72				// we can wait with active spinning.
    73				continue
    74			}
    75			// First store completed. Check type and overwrite data.
    76			if typ != xp.typ {
    77				panic("sync/atomic: store of inconsistently typed value into Value")
    78			}
    79			StorePointer(&vp.data, xp.data)
    80			return
    81		}
    82	}
    83	
    84	// Disable/enable preemption, implemented in runtime.
    85	func runtime_procPin()
    86	func runtime_procUnpin()
    87	

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