lfstack.go

     1  // Copyright 2012 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  // Lock-free stack.
     6  
     7  package runtime
     8  
     9  import (
    10  	"internal/runtime/atomic"
    11  	"unsafe"
    12  )
    13  
    14  // lfstack is the head of a lock-free stack.
    15  //
    16  // The zero value of lfstack is an empty list.
    17  //
    18  // This stack is intrusive. Nodes must embed lfnode as the first field.
    19  //
    20  // The stack does not keep GC-visible pointers to nodes, so the caller
    21  // must ensure the nodes are allocated outside the Go heap.
    22  type lfstack uint64
    23  
    24  func (head *lfstack) push(node *lfnode) {
    25  	node.pushcnt++
    26  	new := lfstackPack(node, node.pushcnt)
    27  	if node1 := lfstackUnpack(new); node1 != node {
    28  		print("runtime: lfstack.push invalid packing: node=", node, " cnt=", hex(node.pushcnt), " packed=", hex(new), " -> node=", node1, "\n")
    29  		throw("lfstack.push")
    30  	}
    31  	for {
    32  		old := atomic.Load64((*uint64)(head))
    33  		node.next = old
    34  		if atomic.Cas64((*uint64)(head), old, new) {
    35  			break
    36  		}
    37  	}
    38  }
    39  
    40  func (head *lfstack) pop() unsafe.Pointer {
    41  	for {
    42  		old := atomic.Load64((*uint64)(head))
    43  		if old == 0 {
    44  			return nil
    45  		}
    46  		node := lfstackUnpack(old)
    47  		next := atomic.Load64(&node.next)
    48  		if atomic.Cas64((*uint64)(head), old, next) {
    49  			return unsafe.Pointer(node)
    50  		}
    51  	}
    52  }
    53  
    54  func (head *lfstack) empty() bool {
    55  	return atomic.Load64((*uint64)(head)) == 0
    56  }
    57  
    58  // lfnodeValidate panics if node is not a valid address for use with
    59  // lfstack.push. This only needs to be called when node is allocated.
    60  func lfnodeValidate(node *lfnode) {
    61  	if base, _, _ := findObject(uintptr(unsafe.Pointer(node)), 0, 0); base != 0 {
    62  		throw("lfstack node allocated from the heap")
    63  	}
    64  	if lfstackUnpack(lfstackPack(node, ^uintptr(0))) != node {
    65  		printlock()
    66  		println("runtime: bad lfnode address", hex(uintptr(unsafe.Pointer(node))))
    67  		throw("bad lfnode address")
    68  	}
    69  }
    70  
    71  func lfstackPack(node *lfnode, cnt uintptr) uint64 {
    72  	return uint64(taggedPointerPack(unsafe.Pointer(node), cnt))
    73  }
    74  
    75  func lfstackUnpack(val uint64) *lfnode {
    76  	return (*lfnode)(taggedPointer(val).pointer())
    77  }
    78
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