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  	"runtime/internal/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  // is responsible for ensuring the nodes are not garbage collected
    22  // (typically by allocating them from manually-managed memory).
    23  type lfstack uint64
    24  
    25  func (head *lfstack) push(node *lfnode) {
    26  	node.pushcnt++
    27  	new := lfstackPack(node, node.pushcnt)
    28  	if node1 := lfstackUnpack(new); node1 != node {
    29  		print("runtime: lfstack.push invalid packing: node=", node, " cnt=", hex(node.pushcnt), " packed=", hex(new), " -> node=", node1, "\n")
    30  		throw("lfstack.push")
    31  	}
    32  	for {
    33  		old := atomic.Load64((*uint64)(head))
    34  		node.next = old
    35  		if atomic.Cas64((*uint64)(head), old, new) {
    36  			break
    37  		}
    38  	}
    39  }
    40  
    41  func (head *lfstack) pop() unsafe.Pointer {
    42  	for {
    43  		old := atomic.Load64((*uint64)(head))
    44  		if old == 0 {
    45  			return nil
    46  		}
    47  		node := lfstackUnpack(old)
    48  		next := atomic.Load64(&node.next)
    49  		if atomic.Cas64((*uint64)(head), old, next) {
    50  			return unsafe.Pointer(node)
    51  		}
    52  	}
    53  }
    54  
    55  func (head *lfstack) empty() bool {
    56  	return atomic.Load64((*uint64)(head)) == 0
    57  }
    58  
    59  // lfnodeValidate panics if node is not a valid address for use with
    60  // lfstack.push. This only needs to be called when node is allocated.
    61  func lfnodeValidate(node *lfnode) {
    62  	if lfstackUnpack(lfstackPack(node, ^uintptr(0))) != node {
    63  		printlock()
    64  		println("runtime: bad lfnode address", hex(uintptr(unsafe.Pointer(node))))
    65  		throw("bad lfnode address")
    66  	}
    67  }
    68
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