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 [Value.Load].
    13  // Once [Value.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 any
    18  }
    19  
    20  // efaceWords is interface{} internal representation.
    21  type efaceWords 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() (val any) {
    29  	vp := (*efaceWords)(unsafe.Pointer(v))
    30  	typ := LoadPointer(&vp.typ)
    31  	if typ == nil || typ == unsafe.Pointer(&firstStoreInProgress) {
    32  		// First store not yet completed.
    33  		return nil
    34  	}
    35  	data := LoadPointer(&vp.data)
    36  	vlp := (*efaceWords)(unsafe.Pointer(&val))
    37  	vlp.typ = typ
    38  	vlp.data = data
    39  	return
    40  }
    41  
    42  var firstStoreInProgress byte
    43  
    44  // Store sets the value of the [Value] v to val.
    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(val any) {
    48  	if val == nil {
    49  		panic("sync/atomic: store of nil value into Value")
    50  	}
    51  	vp := (*efaceWords)(unsafe.Pointer(v))
    52  	vlp := (*efaceWords)(unsafe.Pointer(&val))
    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.
    59  			runtime_procPin()
    60  			if !CompareAndSwapPointer(&vp.typ, nil, unsafe.Pointer(&firstStoreInProgress)) {
    61  				runtime_procUnpin()
    62  				continue
    63  			}
    64  			// Complete first store.
    65  			StorePointer(&vp.data, vlp.data)
    66  			StorePointer(&vp.typ, vlp.typ)
    67  			runtime_procUnpin()
    68  			return
    69  		}
    70  		if typ == unsafe.Pointer(&firstStoreInProgress) {
    71  			// First store in progress. Wait.
    72  			// Since we disable preemption around the first store,
    73  			// we can wait with active spinning.
    74  			continue
    75  		}
    76  		// First store completed. Check type and overwrite data.
    77  		if typ != vlp.typ {
    78  			panic("sync/atomic: store of inconsistently typed value into Value")
    79  		}
    80  		StorePointer(&vp.data, vlp.data)
    81  		return
    82  	}
    83  }
    84  
    85  // Swap stores new into Value and returns the previous value. It returns nil if
    86  // the Value is empty.
    87  //
    88  // All calls to Swap for a given Value must use values of the same concrete
    89  // type. Swap of an inconsistent type panics, as does Swap(nil).
    90  func (v *Value) Swap(new any) (old any) {
    91  	if new == nil {
    92  		panic("sync/atomic: swap of nil value into Value")
    93  	}
    94  	vp := (*efaceWords)(unsafe.Pointer(v))
    95  	np := (*efaceWords)(unsafe.Pointer(&new))
    96  	for {
    97  		typ := LoadPointer(&vp.typ)
    98  		if typ == nil {
    99  			// Attempt to start first store.
   100  			// Disable preemption so that other goroutines can use
   101  			// active spin wait to wait for completion.
   102  			runtime_procPin()
   103  			if !CompareAndSwapPointer(&vp.typ, nil, unsafe.Pointer(&firstStoreInProgress)) {
   104  				runtime_procUnpin()
   105  				continue
   106  			}
   107  			// Complete first store.
   108  			StorePointer(&vp.data, np.data)
   109  			StorePointer(&vp.typ, np.typ)
   110  			runtime_procUnpin()
   111  			return nil
   112  		}
   113  		if typ == unsafe.Pointer(&firstStoreInProgress) {
   114  			// First store in progress. Wait.
   115  			// Since we disable preemption around the first store,
   116  			// we can wait with active spinning.
   117  			continue
   118  		}
   119  		// First store completed. Check type and overwrite data.
   120  		if typ != np.typ {
   121  			panic("sync/atomic: swap of inconsistently typed value into Value")
   122  		}
   123  		op := (*efaceWords)(unsafe.Pointer(&old))
   124  		op.typ, op.data = np.typ, SwapPointer(&vp.data, np.data)
   125  		return old
   126  	}
   127  }
   128  
   129  // CompareAndSwap executes the compare-and-swap operation for the [Value].
   130  //
   131  // All calls to CompareAndSwap for a given Value must use values of the same
   132  // concrete type. CompareAndSwap of an inconsistent type panics, as does
   133  // CompareAndSwap(old, nil).
   134  func (v *Value) CompareAndSwap(old, new any) (swapped bool) {
   135  	if new == nil {
   136  		panic("sync/atomic: compare and swap of nil value into Value")
   137  	}
   138  	vp := (*efaceWords)(unsafe.Pointer(v))
   139  	np := (*efaceWords)(unsafe.Pointer(&new))
   140  	op := (*efaceWords)(unsafe.Pointer(&old))
   141  	if op.typ != nil && np.typ != op.typ {
   142  		panic("sync/atomic: compare and swap of inconsistently typed values")
   143  	}
   144  	for {
   145  		typ := LoadPointer(&vp.typ)
   146  		if typ == nil {
   147  			if old != nil {
   148  				return false
   149  			}
   150  			// Attempt to start first store.
   151  			// Disable preemption so that other goroutines can use
   152  			// active spin wait to wait for completion.
   153  			runtime_procPin()
   154  			if !CompareAndSwapPointer(&vp.typ, nil, unsafe.Pointer(&firstStoreInProgress)) {
   155  				runtime_procUnpin()
   156  				continue
   157  			}
   158  			// Complete first store.
   159  			StorePointer(&vp.data, np.data)
   160  			StorePointer(&vp.typ, np.typ)
   161  			runtime_procUnpin()
   162  			return true
   163  		}
   164  		if typ == unsafe.Pointer(&firstStoreInProgress) {
   165  			// First store in progress. Wait.
   166  			// Since we disable preemption around the first store,
   167  			// we can wait with active spinning.
   168  			continue
   169  		}
   170  		// First store completed. Check type and overwrite data.
   171  		if typ != np.typ {
   172  			panic("sync/atomic: compare and swap of inconsistently typed value into Value")
   173  		}
   174  		// Compare old and current via runtime equality check.
   175  		// This allows value types to be compared, something
   176  		// not offered by the package functions.
   177  		// CompareAndSwapPointer below only ensures vp.data
   178  		// has not changed since LoadPointer.
   179  		data := LoadPointer(&vp.data)
   180  		var i any
   181  		(*efaceWords)(unsafe.Pointer(&i)).typ = typ
   182  		(*efaceWords)(unsafe.Pointer(&i)).data = data
   183  		if i != old {
   184  			return false
   185  		}
   186  		return CompareAndSwapPointer(&vp.data, data, np.data)
   187  	}
   188  }
   189  
   190  // Disable/enable preemption, implemented in runtime.
   191  func runtime_procPin() int
   192  func runtime_procUnpin()
   193
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