Abstract:
Multiple non-blocking FIFO queues are concurrently maintained using atomic compare-and-swap (CAS) operations. In accordance with the invention, each queue provides direct access to the nodes stored therein to an application or thread, so that each thread may enqueue and dequeue nodes that it may choose. The prior art merely provided access to the values stored in the node. In order to avoid anomalies, the queue is never allowed to become empty by requiring the presence of at least a dummy node in the queue. The ABA problem is solved by requiring that the next pointer of the tail node in each queue point to a “magic number” unique to the particular queue, such as the pointer to the queue head or the address of the queue head, for example. This obviates any need to maintain a separate count for each node.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field  
           [0002]    The invention is related to first-in-first-out (FIFO) queues employing non-blocking atomic compare-and-swap (CAS) instructions.  
           [0003]    2. Background Art  
           [0004]    A FIFO queue may be used by various application or process threads which may wish to enqueue or dequeue certain data on the queue. Typically, a queue is a list of different memory locations containing particular data, and each memory location is typically referred to as a “node” of the queue. The nodes are kept in order by providing in each node a “next” pointer that points to the memory location of the next node in the queue. The head of the queue is the first node (“head node”) while the last node is the tail node. The tail node&#39;s next pointer points to a predetermined number, such as NULL. A node is enqueued by inserting it at the tail so that it becomes the new tail node of the queue. This requires the thread to first determine which node is the current tail node. Nodes are dequeued at the head, so that the head node is dequeued and the next node becomes the head node. This requires the thread to first determine which node is the current head node. The queue has a head pointer pointing to the head node and a tail pointer pointing to the tail node.  
           [0005]    Maintaining the integrity of the queue while permitting its concurrent use by a number of different threads is a difficult problem. To solve this problem, the queue design must address all possible pathological conditions that the queue could experience. For example, after one thread has identified the tail node in preparation for an enqueue operation, another thread may interrupt and enqueue another node onto the queue (which obsoletes the one node&#39;s prior identification of the tail node). As another example: the head and tail nodes may be one and the same node because it is the only node on the queue; and one thread may identify the tail node in preparation for enqueueing a new node onto the queue; but, before it can, another thread may dequeue and move the tail node to another queue (for example) without changing its next pointer from NULL. In this case, the one thread may still succeed in attaching the new node to what it still believes is the tail node of the desired queue, but would actually be enqueueing the new node on the wrong queue. This latter case is typically referred to as the “ABA problem” and is described extensively in the literature. It is plausible that such an event could occur even if there were more than one node on the queue in the following example: after the one thread identifies the tail node, actions by other threads cause the tail node to be moved to the head and then dequeued and re-enqueued on another queue before the one thread completes its enqueueing operation. In any case, the ABA problem entails the risk of a thread unknowingly enqueueing a new node on the wrong queue or other location.  
           [0006]    Initially, the ABA problem was solved by providing, whenever one thread was in the middle of an enqueue or dequeue operation, a lock which protected the queue from being changed by another contending thread. However, such blocking queues are susceptible to large unpredictable delays in process execution, since a single thread can monopolize the queue, particularly if it is a low priority thread that is interrupted by other higher priority threads.  
           [0007]    As a result, the art has sought a non-blocking queue (i.e., a queue with no locks) permitting concurrent access to the queue by more than one thread without suffering failures due to the ABA problem. In such a concurrent non-blocking queue, the ABA problem has been solved in ways that burden the queue and impair performance. One such concurrent non-blocking queue is described by Michael et al., “Simple, Fast, and Practical Non-Blocking and Blocking Concurrent Queue Algorithms,”  PODC,  1996. This publication describes a concurrent non-blocking queue in which the ABA problem is addressed by assigning an extra “count” field to the queue pointers such as the next pointer of the tail node. Thus, for example, each time the tail node is modified by any thread, the count associated with the next pointer of the tail node would be incremented. In the ABA situation, if the tail node has been dequeued and re-enqueued on another node, a thread trying to enqueue a new node onto the first queue would recognize that the next pointer “count” field of the what it believes to be tail node has changed, even if the next pointer still has the same value as before. Therefore the thread would not complete its enqueue operation, thereby preventing an ABA problem.  
           [0008]    Another difficulty in the implementation of a non-blocking queue is the method of handling the case where the queue is empty; in other words, when there are no nodes in the queue. Support for enqueueing a node on an empty queue, or dequeueing the last node on a queue (leaving it empty) can greatly complicate the implementation, as each enqueue and dequeue operation would then need to maintain both the head and tail pointers. To simplify this case, the queue in the Michael publication keeps at least one node in the queue at all times. To implement this, the queue in the Michael publication must control the nodes, rather than letting threads enqueue or dequeue their own nodes. In the Michael publication, each node is selected from a list maintained for the queue. The data of interest is then stored in the node. Such data is taken from a thread and copied into the node for an “enqueue” operation. It is later copied out of the node and returned to a thread for a “dequeue” operation while the node itself is not, the node always being preserved for use with the queue. If the dequeue operation determines that the node being dequeued is the last node in the queue, it is left there to ensure that there is always at least one node in the queue.  
           [0009]    The requirement that the queue allocate and deallocate the individual nodes constricts queue performance and constricts the manner in which threads may use the queue. This is especially true with regard to situations where the enqueue or dequeue operations may take place in an execution context from which memory allocation operations cannot be invoked (such as within an interrupt handler).  
           [0010]    It is therefore desired to provide a concurrent non-blocking queue in which it is not necessary to maintain extra count fields and in which the threads themselves enqueue and dequeue any nodes they wish on the queue without any risk of emptying the queue.  
         SUMMARY OF THE DISCLOSURE  
         [0011]    The design described here differs from the Michael publication in two fundamental ways:  
           [0012]    a) The use of a “magic number” (other than NULL) to be placed into the next pointer of the last node in the list, thus avoiding the use of a count and circumventing the ABA problem  
           [0013]    b) The use of a dummy node to ensure that the queue is never empty, while still allowing the enqueue and dequeue of nodes managed outside of the control of the queue itself.  
           [0014]    An application or thread enqueues a new node into the queue by, first, setting the next pointer of the new node to the magic number. If the next pointer of the current tail node points to the magic number, then its next pointer is changed to point to the new node. If this operation is successful, then the queue&#39;s tail pointer is changed to point to the new node. If the foregoing conditions were not satisfied, then the tail pointer has been moved by another application or thread during the interim. This is corrected by changing the tail pointer to the next pointer of the node currently pointed to by the tail pointer. Then, the enqueue process is attempted again, and this cycle is repeated until successful.  
           [0015]    An application or thread dequeues a node from the queue by, first, making local copies of the current version of the queue&#39;s head pointer, tail pointer and the next pointer of the head node (the node pointed to by the head pointer). A check is then made to ensure that the queue&#39;s head pointer has not changed, and then a check is made to ensure that the head and tail pointers do not point to the same thing. If they do, this indicates that either (a) the queue is empty or (b) another thread has changed the queue so that the tail pointer needs correcting. These two possibilities are resolved by checking whether the next pointer of the head node points to the magic number (in which case the queue is empty). If the queue is not empty, the tail pointer is corrected by changing it to point to the node pointed to by the next pointer of the node currently pointed to by the tail pointer. The foregoing dequeue process is then repeated until the above conditions are met. Once the above conditions are met (i.e., the head and tail pointers do not point to the same node), the current head node is dequeued by changing the head pointer to point to the node currently pointed to by the next pointer of the node being dequeued. Next, the dequeued node is checked to ensure that it is not the dummy node. If it is, then the dummy node is re-enqueued and the next node is dequeued as the one actually desired by the application.  
           [0016]    In accordance with one aspect of the invention, a method is provided for one thread in a system running plural threads to enqueue a new node of its own choosing onto a selected FIFO queue, the system having plural FIFO queues, each queue including a succession of enqueued nodes and having a head pointer pointing to a head node and a tail pointer pointing to a tail node, each of the nodes having a next pointer, the next pointers of the enqueued nodes pointing to the next node in the succession from the head node to the tail node. The enqueueing method is carried out by first obtaining from the selected queue a queue-specific number of the selected queue unique to the selected queue. In this embodiment, this queue-specific number is used as the “magic number”. The next step is setting the next pointer of the new node to the queue-specific number. A determination is next made as to whether another one of the threads has preempted the one thread and, if so, updating the tail if needed and then re-starting the method. Otherwise, the next step is setting the next pointer of the tail node to point to the new node. The final step is setting the tail pointer to point to the new node if it has not been updated by another thread during the execution of the enqueueing method.  
           [0017]    The step of determining whether another one of the threads has preempted the one thread includes making a local copy of the tail pointer of the selected queue and then determining whether the next pointer of the tail node of the selected queue no longer points to the queue-specific number of the selected queue. If the next pointer no longer points to the queue-specific number, a determination is made as to whether the tail pointer of the selected queue has changed since the local copy of the tail pointer was made.  
           [0018]    The step of updating the tail pointer is needed if the tail pointer has not changed since the local copy was made, and is performed by changing the tail pointer to be equal to the next pointer of the tail node of the selected queue.  
           [0019]    The step of setting the tail pointer to the new node if it has not been updated by another thread is carried out by first determining whether the tail pointer of the selected queue has not changed since the making of the local copy. If the tail pointer has not changed since the making of the local copy, the tall pointer is changed by setting the tail pointer to point to the new node.  
           [0020]    In the general case, the next pointer of the tail node of the queue initially points to the queue-specific number. The queue-specific number may be the address of the head pointer of the queue or the address of the tail pointer of the queue or a pointer having its low bit set to one or a system-wide unique identifier that is assigned to the queue at creation time, or some combination of the above, for example.  
           [0021]    A dummy node having a next pointer is always present (although it may be temporarily dequeued by a thread). The next pointer of the dummy node points to a next node in the queue if the dummy is not currently the tail node and points to the queue-specific number if the queue is empty. In this way, the queue always contains at least one node.  
           [0022]    In accordance with another aspect of the invention, a method is provided for one thread in a system running plural threads to dequeue a node from a selected one of the FIFO queues. The method is performed by first determining whether another thread has preempted the one thread and dequeued a node from the head of the queue and, if so, re-starting the method. Otherwise, the next step is determining, in the event the queue appears to be empty, whether another thread has preempted the one thread by enqueueing a new node at the tail of the queue, and if the other thread did not update the tail pointer, updating the tail pointer and re-starting the method. If the queue does not appear to be empty, the next step is determining whether another thread has preempted the one thread and dequeued a node from the head of the queue and, if so, re-starting the method. Otherwise, the head node is dequeued by changing the head pointer to equal the next pointer of the head node. Finally, if the dequeued node is a dummy node, the dummy node must be re-enqueued onto the queue. At this point, the thread may restart the dequeueing method with the new head node.  
           [0023]    The step of determining whether another thread has preempted the one thread is preceded by first determining whether the queue appears to be empty. This is accomplished by determining whether the head pointer and the tail pointer point to the same node. If so, it is then determined whether the queue is actually empty by determining whether the next pointer of the head node points to the queue-specific number. If this is the case, the queue is considered empty and the operation is terminated.  
           [0024]    The step of determining whether another thread has preempted the one thread and dequeued a node from the head is preceded by making a local copy of the head pointer, the tail pointer and the next pointer of the head node. The step of determining whether another thread has preempted the one thread and dequeued a node from the head consists of determining whether the head pointer has changed since the making of the local copy. The step of determining whether another thread has preempted the one thread and enqueued a new node at the tail consists of determining whether the tail pointer has changed since the making of the local copy. The step of determining whether the queue is empty consists of determining whether the next pointer of the head node is the queue-specific number. The step of updating the tail pointer consists of changing the tail pointer to equal the next pointer of the tail node (i.e., the node currently pointed to by the tail pointer).  
           [0025]    In accordance with a further aspect of the invention, a method is provided for constructing a FIFO queue data structure. This method is carried out by first providing memory space for a head pointer, a tail pointer and a dummy node. Initially, the new queue will contain only the dummy node. The next step is to set the head pointer to point to the dummy node, set the tail pointer to pointer to the dummy node and set the next pointer of the dummy node to point to the queue-specific nurser.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    [0026]FIG. 1 illustrates an exemplary operating environment of the invention.  
         [0027]    [0027]FIG. 2 is a block diagram of apparatus embodying an aspect of the invention and illustrating an enqueue operation.  
         [0028]    [0028]FIG. 3 is a flow diagram of an enqueue operation carried out in the apparatus of FIG. 2.  
         [0029]    [0029]FIG. 4 is a block diagram of apparatus embodying an aspect of the invention and illustrating a dequeue operation.  
         [0030]    [0030]FIG. 5 is a flow diagram of a dequeue operation carried out in the apparatus of FIG. 4.  
         [0031]    [0031]FIG. 6 is a diagram illustrating a queue interface object embodying one aspect of the invention.  
         [0032]    [0032]FIG. 7 is a flow diagram illustrating a process of the constructing the queue interface object of FIG. 6.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]    Exemplary Operating Environment  
         [0034]    [0034]FIG. 1 and the following discussion are intended to provide a brief, general description of a suitable computing environment in which the invention may be implemented. Although not required, the invention will be described in the general context of computer-executable instructions, such as program, modules, being executed by a personal computer. Generally, program modules include processes, programs, objects, components, data structures, etc. that perform particular tasks or implement particular abstract data types. Moreover, those skilled in the art will appreciate that the invention may be practiced with other computer system configurations, including inside various programmable peripheral interface cards such as  126 ,  128 ,  130 ,  144 ,  158 ,  148  in FIG. 1, inside programmable peripherals such as disks, game controllers and accessories, speakers, modems, printers and the like, in hand-held devices, multiprocessor systems, microprocessor-based or programmable consumer electronics, network PCs, minicomputers, mainframe computers, and the like. Thus, for example, the present invention can be an operating system of an optionally minimized configuration, as described below, running inside a network interface card of the network interface  158  of FIG. 1 or in an embedded control system or in a communication-oriented device. The invention may also be practiced in distributed computing environments where tasks are performed by remote processing devices that are linked through a communications network. In a distributed computing environment, program modules may be located both in local and in remote memory storage devices.  
         [0035]    With reference to FIG. 1, an exemplary system for implementing the invention includes a general purpose computing device in the form of a conventional personal computer  120 , including a processing unit  121 , a system memory  122 , and a system bus  123  that couples various system components including the system memory to the processing unit  121 . The system bus  123  may be any of several types of bus structures including a memory bus or memory controller, a peripheral bus, and a local bus using any of a variety of bus architectures. The system memory includes read only memory (ROM)  124  and random access memory (RAM)  125 . A basic input/output system  126  (BIOS), containing the basic process that helps to transfer information between elements within the personal computer  120 , such as during start-up, is stored in ROM  124 . The personal computer  120  further includes a hard disk drive  127  for reading from and writing to a hard disk, not shown, a magnetic disk drive  128  for reading from or writing to a removable magnetic disk  129 , and an optical disk drive  130  for reading from or writing to a removable optical disk  131  such as a CD ROM or other optical media. The hard disk drive  127 , magnetic disk drive  128 , and optical disk drive  130  are connected to the system bus  123  by a hard disk drive interface  132 , a magnetic disk drive interface  133 , and an optical drive interface  134 , respectively. The drives and their associated computer-readable media provide nonvolatile storage of computer readable instructions, data structures, program modules and other data for the personal computer  120 . Although the exemplary environment described herein employs a hard disk, a removable magnetic disk  129  and a removable optical disk  131 , it should be appreciated by those skilled in the art that other types of computer readable media which can store data that is accessible by a computer, such as magnetic cassettes, flash memory cards, digital video disks, Bernoulli cartridges, random access memories (RAMs), read only memories (ROM), and the like, may also be used in the exemplary operating environment.  
         [0036]    A number of program modules may be stored on the hard disk, magnetic disk  129 , optical disk  131 , ROM  124  or RAM  125 , including an operating system  135 , one or more application programs  136 , other program modules  137 , and program data  138 . A user may enter commands and information into the personal computer  120  through input devices such as a keyboard  190  and pointing device  142 . Other input devices (not shown) may include a microphone, joystick, game pad, satellite dish, scanner, or the like. These and other input devices are often connected to the processing unit  121  through a serial port interface  146  that is coupled to the system bus, but may be connected by other interfaces, such as a parallel port, game port or a universal serial bus (USB). A monitor  147  or other type of display device is also connected to the system bus  123  via an interface, such as a video adapter  148 . In addition to the monitor, personal computers typically include other peripheral output devices (not shown), such as speakers and printers.  
         [0037]    The personal computer  120  may operate in a networked environment using logical connections to one or more remote computers, such as a remote computer  149 . The remote computer  149  may be another personal computer, a server, a router, a network PC, a peer device or other common network node, and typically includes many or all of the elements described above relative to the personal computer  120 , although only a memory storage device  150  has been illustrated in FIG. 1. The logical connections depicted in FIG. 1 include a local area network (LAN)  151  and a wide area network (WAN)  152 . Such networking environments are commonplace in offices, enterprise-wide computer networks, intranets and Internet.  
         [0038]    When user in a LAN networking environment, the personal computer  120  is connected to the local network  151  through a network interface or adapter  153 . When used in a WAN networking environment, the personal computer  120  typically includes a modem  154  or other means for establishing communications over the wide area network  152 , such as the Internet. The modem  154 , which may be internal or external, is connected to the system bus  123  via the serial port interface  146 . In a networked environment, program modules depicted relative to the personal computer  120 , or portions thereof, may be stored in the remote memory storage device. It will be appreciated that the network connections shown are exemplary and other means of establishing a communications link between the computers may be used.  
         [0039]    Queue Structure  
         [0040]    Referring to FIG. 2, a FIFO queue  200  embodying the present invention consists of a stored list specifying a succession of nodes  205 ,  206 ,  207 ,  208  which are locations in a memory containing data such as the RAM  112  of FIG. 1. The node  206  is a dummy node which is not available to external threads. With the exception of the dummy node  206 , all of the nodes have been enqueued by external process or application threads and any of them may be-dequeued by a thread. The list is specified as follows: each node  205 ,  206 ,  207 ,  208  has a next pointer  205   a,    206   a,    207   a,    208   a,  respectively pointing to the next node in the queue; a head pointer  210  points to the node  205  at the head of the queue (the “head node”) and a tail pointer  212  points to the node  208  at the tail of the queue (the “tail node”). The nodes  205 ,  206 ,  207 ,  208  and their next pointers  205   a,    206   a,    207   a,    208   a,  and the head and tail pointers  210 ,  212  are components of a queue interface (IQueue) object stored in working memory, such as the RAM  112  of FIG. 1. The next pointer of the tail node  212  is a “magic number” or queue-specific number  214  which is unique to the queue  200  relative to any other queue, for example the queue  216 . The queue  216  can have the same structure as the queue  200 . The queue-specific number may be the address of the head pointer  210  of the queue  200  or the address of the tail pointer  212  or a similar pointer with the low bit set to  1  rendering it an invalid pointer which would render it unique relative to any other pointer. It could also be a system-wide unique identifier dynamically generated at queue creation time. Likewise, a different queue-specific number would be similarly employed in the other queue  216 .  
         [0041]    Enqueuing Method  
         [0042]    As will be described below, an IQueue object provides a method for enqueueing a node onto the queue and a method for dequeueing a node from the queue. The node itself is not part of the IQueue object. The method for enqueueing a node will now be described with reference to an example in which a thread  220  enqueues a new node  222  onto the queue  200 . The new node  222  has a next pointer  222   a.  The solid line configuration of FIG. 2 illustrates the state of the queue  200  prior to the new node  222  being enqueued. The dashed lines indicate the changes made in enqueueing the new node  222 . Referring now to FIG. 3, the enqueueing method begins by the thread  220  deciding upon the queue  200  and the new node  222  as the one to be enqueued (block  305  of FIG. 3). Then, the thread  220  sets the next pointer  222   a  of the new node  222  to the queue-specific number  214  (block  310 ). The thread  220  reads the tail pointer  212  and makes a local copy  242 - 1  of the tail pointer  212  (block  315 ). The local copy  212 - 1  cannot be changed by another thread, while the tail pointer  212  in the queue  200  may be changed by some other thread (e.g., the thread  226 ) by preempting the thread  220 , or by running concurrently on a different processor of a shared-memory based multiprocessor computer system.  
         [0043]    A determination is then made as to whether the tail node&#39;s next pointer  208   a  is the queue-specific number  214  (block  320 ). If the comparison fails (NO branch of block  320 ), this means that sometime after the local copy  212 - 1  was made, some other thread (e.g., the thread  226 ) enqueued onto the tail of the queue  200  another node unknown to the thread  200 . In such an event, the next pointer  208   a  would have been changed by the other thread to point to the “unknown” node it enqueued. (The “unknown” node enqueued by the other thread in this event is not illustrated in FIG. 3 for the sake of simplicity). In this event, the other thread might have failed to update the tail pointer  212  so that the tail pointer  212  needs updating. Therefore the next step is to determine whether the tail pointer  212  has changed since the local copy  224  was made (block  325 ). If not (NO branch of block  325 ), the tail pointer  212  is updated by changing the tail pointer  212  to be the same as the next pointer  208   a  of what used to be the tail node  208  (block  330 ). Otherwise, if the tail pointer  212  has changed (YES branch of block  325 ), the updating step (block  330 ) is skipped and the process returns to the beginning (e.g., to block  315 ) to make a next attempt by repeating the foregoing steps of blocks  315 - 330 . After one or more such attempts, the determination step of block  320  will ultimately succeed (YES branch of block  320 ). In this case, what is now the current tail node&#39;s next pointer  208   a  is changed to point to the new node  222  being enqueued by the thread  220  (block  335 ). This step is indicated in FIG. 2 showing the arrow from the next pointer  208   a  swinging from its solid line position to the dashed line position. The new node  222  is now in the queue  200 . The next step is to update the tail pointer  212  to point to the new node  222 . However, another thread may have just enqueued another node (unknown to the thread  220 ), and updated the tail pointer  212  accordingly, it which case the tail pointer  212  should not be changed. Therefore, a determination is first made to be sure that some other thread has not changed the tail pointer  212  since the local copy was made (block  340 ). If so (YES branch of block  340 ), the tail pointer  212  is updated to point to the new node  222  (block  345 ). This is illustrated in FIG. 2 showing the arrow from the tail pointer  212  swinging from its solid line position to the dashed line position. Otherwise (NO breach of block  340 ), if the tail pointer  212  has changed the step of block is skipped and the enqueueing method is finished. Since many different threads (e.g., the threads  220 ,  226 ) have concurrent access to anyone of the queues  200 ,  216 , each queue is a concurrent queue.  
         [0044]    The foregoing enqueue method may be summarized as follows: A thread designates of its own choosing any node to which it has access for enqueueing into the queue. The next pointer of the node to be enqueued is set to the queue-specific number (block  310 ) and the tail pointer is tested for correctness (block  315 ,  320 ). An attempt is made to correct the tail pointer if incorrect (blocks  325 ,  330 ) and the test repeated. Otherwise, if the tail pointer is correct, the next pointer of the current tail node is swung to point to the new node (block  335 ) and the tail pointer is updated accordingly unless another thread has intervened to change the tail pointer (blocks  340 ,  345 ).  
         [0045]    Dequeueing Method  
         [0046]    [0046]FIG. 4 illustrates changes to the queue structure of FIG. 2 that are made in dequeueing a node from the queue  200 . In general, a successful dequeue operation will remove the head node  205  by swinging the head pointer  210  from the current head node  205  to its successor, the node  206 . This is illustrated in FIG. 4 by the arrow from the head pointer  210  swinging from its solid line position to the dashed line position. The dequeueing method of the invention will now be described with reference to FIG. 5.  
         [0047]    An important feature of the dequeue method of the invention is that no thread is allowed to dequeue the dummy node  206 , in that whenever the dummy node reaches the head of the queue and is dequeued, the method requires the thread holding the dummy node  206  to re-enqueue the dummy node  20  sing the enqueue process described above with reference to FIGS. 2 and 3. Moreover, no thread is permitted to dequeue any node from the queue if it is the one node remaining in the queue. This feature enables the queue methods of the invention to permit threads to directly enqueue and dequeue nodes they may choose and actually remove a dequeued node from the queue, rather than merely access the data stored in the node. By always requiring at least one node to be present in the queue, e.g., the dummy node  206 , the head and tail pointers  210 ,  212  always have a node in the queue to point to and the structure is simple and reliable, a significant advantage.  
         [0048]    The dequeue begins with the thread  220  deciding upon the queue  200  as one from which to dequeue a node (block  505  of FIG. 5). Normally, the thread  220  simply wishes to retrieve the first element of the queue, e.g. the next work item in a list of such. In an alternative embodiment of the present invention, the thread  220  may be looking for a particular node it believes to be on the queue  200  and which it therefore desires to dequeue. If this node is not currently the head node, then the thread will have to wait until it becomes so, or it may dequeue and re-enqueue successive nodes of the queue until the desired node becomes the head node  205 .  
         [0049]    Dequeueing the first node in the queue is accomplished as follows. The thread  220  first makes local copies of the queue&#39;s head pointer  210 , tail pointer  212  and of the next pointer  205   a  of the current head node  205  (block  510 ). These local copies are illustrated in FIG. 4 as the local copies  210 - 1 ,  212 - 1  and  205   a - 1  in the thread  220 . Next, a “stability” check is performed by determining whether the head pointer  210  has changed since the local copy  210 - 1  was made (block  515 ). If so (YES branch of block  515 ), another thread (e.g., the thread  226 ) has preempted the thread  220 , and the process must return to the beginning (block  510 ). Otherwise ,NO branch of block  515 ), the queue has not changed and the dequeueing method may continue with the next step, which is determining whether or not the head and tail pointers  210 ,  212  point to the same node (block  520 ).  
         [0050]    The test of block  520  is made because one of two conditions may be present that would affect the dequeue method: (1) there may be only one node in the queue (e.g., the dummy node  206 ), in which case no dequeue operation is allowed in order to prevent the queue from becoming completely empty, or (2) the queue is not empty but the tail pointer  212  does not point to the current tail node. In condition (1) (only one node in the queue), the one remaining node would typically be the dummy node  206 , unless it has been dequeued by another thread, in which the other thread is waiting to return the dummy node to the queue, as will be described below. Condition (2) may arise by another thread, while preempting the thread  220 , enqueueing a new node but failing to update the tail pointer  212 . With condition (1), the dequeue operation must be terminated to keep at least one node in the queue, while with condition (2) the tail pointer  212  should be updated and the dequeueing operation allowed to continue. In order to distinguish between conditions (1) and (2), a determination is made whether the head node&#39;s next pointer  205   a  is the queue-specific number  214  (block  525 ). It does (YES branch of block  525 ), there is only one remaining node in the queue, and the queue process is terminated in order to avoid completely emptying the queue (block  530 ). Otherwise (NO branch of block  525 ), there is more than one node on the queue and (local copies of) the head and tail pointers are the same just because the tail pointer  212  is wrong. This indicates that another thread has probably intervened to enqueue a new node, so that there are at least two nodes on the queue. Furthermore, yet another thread may then intervene and set the tail pointer  212  to the true tail node which it just enqueued, in which case the tail pointer  212  might now be correct and should not be changed. Therefore, a determination is first made as to whether the tail pointer  212  has changed since the local copy  212 - 1  was made (block  535 ). If not (NO branch of block  535 ), the tail pointer  212  is set to equal the next pointer of what the local copy  212 - 1  identified as the tail node (and which is no longer the real tail node due to the intervention by another thread) (block  540 ). Otherwise (YES branch of block  535 ), the tail pointer correction step of block  540  is skipped. In either case, the entire process is restarted (at the beginning of the step of block  510 ) for a next attempt to dequeue. This loop is reiterated until the determination step of block  520  finds that the head and tail pointers  210 ,  215  point to different nodes (YES branch of block  520 ). This means that the tail node has not been changed and now it must be determined whether the head node has changed. Thus, the next step is to determine whether the head pointer  210  has changed since the local copy  210 - 1  was made (block  545 ). If it has changed (YES branch of block  545 ), another thread has probably intervened and pulled a node off the queue, and therefore the entire dequeue process must be restarted (back to he beginning of the step of block  510 ). Otherwise (NO branch of block  545 ), no other thread has intervened and the dequeue operation may be carried out to completion. Thus, the next step is to change the head pointer  210  to equal the contents of the next pointer  205   a  of the head node  205 , so that the next node  206  becomes the new head node (block  550 ). This change is illustrated in FIG. 4 with the arrow from the next pointer  205   a  swinging from its solid line position to the dashed line position. In order to avoid losing the dummy node from the queue, the next step is to check whether the dequeued node is the dummy node (block  555 ). If it is (YES branch of block  555 ), then the thread must re-enqueue the dummy node back onto the queue  200  using the enqueue method of FIG. 3 (block  560 ), and return to the beginning of the dequeue method (to the step of block  510 ). Otherwise (NO branch of block  555 ) the dequeue operation has successfully finished and the node  205  has been dequeued from the queue  200 .  
         [0051]    The foregoing dequeue method may be summarized as follows: A thread specifies of its own choosing any queue from which it desires to remove the first node. It then checks to see whether another thread has changed the head pointer (block  515 ) and if so the method is re-started. Otherwise, it determines whether the tail pointer is anomalous (block  520 ). If so, it determines whether the tail pointer needs correction or whether the queue is empty (block  525 ). If the queue is empty, the method is terminated. Otherwise, an attempt is made to correct the tail pointer (blocks  535 ,  540 ) and the method is re-started. On the other hand, if the tail pointer is not anomalous, a stability check of the head pointer is made and the head is swung to away from the head node to the second node provided the head pointer has not changed (blocks  545 ,  500 ), which dequeues the head node. However, if the dequeued node is the dummy, it is re-enqueued and the operation re-started (block  560 ).  
         [0052]    Constructing the IQueue Object  
         [0053]    The queue  200  and its enqueueing and dequeueing methods may be provided as a loadable object such as a component object model (COM) object having an interface by which its methods are made available to other threads or objects. Such an object is illustrated in FIG. 6 and includes a queue object  610  with an IQueue instance pointer  615  and a V table pointer  620  to a set of methods  625 . The queue object  610  includes the head pointer  210 , the tail pointer  215 , the dummy node  206   a  which only needs to contain the next pointer field. The set of methods  625  includes the typical COM object methods of QueryInterface  630 , AddReference  635  and DeleteReference  640 . In addition, the set of methods  625  includes the enqueue method  645  of FIG. 3 and the dequeue method  650  of FIG. 5. Each of these methods has a method pointer to an appropriate implementation containing the code for carrying out the method. Thus, the enqueue method provides a pointer to an implementation containing executable instructions or code corresponding to the flow chart of FIG. 3. The dequeue method provides a pointer to an implementation containing executable instructions or code corresponding to the flow chart of FIG. 5. The query interface method, as in a typical COM object, permits any thread having an IUknown pointer to the object to ask the object for a particular interface (such as IUnknown or IQueue). Such COM interfaces are discussed in U.S. application Ser. No. 08,282,238 filed Mar. 31, 1999 by Raffman et al. and entitled “A Highly Componentized System Architecture with a Demand-Loading Namespace and Programming Model”, the disclosure of which is hereby incorporated by reference.  
         [0054]    The IQueue object of FIG. 6 has a constructor for constructing a specific queue, and the constructor operates in the manner illustrated in FIG. 7. The first step carried out by the constructor is to define the queue structure (block  710  of FIG. 7). This includes constructing a V table pointer, a head pointer, a tail pointer, a dummy node, a dummy node next pointer, and an IQueue instance pointer. Note that the queue-specific number is preferably computed inside the Enqueue and Dequeue methods and does not need to occupy memory storage. In an alternate embodiment of the present invention a queue-specific number might also be defined. The next step is to initialize the structure (block  720 ) as follows: Set the head pointer to point to the dummy node (block  722 ). Set the tail pointer to point to the dummy node (block  723 ). And, set the next pointer of the dummy node to the queue-specific number (block  724 ).  
         [0055]    In a preferred implementation, while each next pointer ( 205   a,    206   a,  etc.) is 32 bits, the head pointer  210  and the tail pointer  212  are each 64 bits, of which 32 bits are address bits and 32 bits are used for a version number.  
         [0056]    In carrying out this preferred implementation in the process of FIG. 3, when the tail pointer is changed in the step of block  330  of FIG. 3, its 32 bit address field is changed to the tail node&#39;s next pointer, and, in addition, its 32 bit version field is incremented. Thus, in the step of block  325  of FIG. 3, in determining whether the tail pointer has changed, both the 32 bit address field and the 32 bit version field are compared with the stored version of the tail pointer. If either the address or the version field has changed, then the conclusion is that the tail pointer has changed.  
         [0057]    In carrying out this preferred implementation in the process of FIG. 5, the step of block  540  of changing the tail pointer involves changing the 32 bit address field and incrementing the 32 bit version field, as described above with reference to the process of FIG. 3. Thus, the step of block  535  of FIG. 5 determines whether the tail pointer has changed by determining whether the address field has changed and whether the version field has changed, as described above with reference to FIG. 3. Similarly in FIG. 5, the step of block  550  of changing the head pointer involves changing the head pointer&#39;s 32 bit address field as well as incrementing the head pointer&#39;s 32 bit version field. Thus, the step of block  545  of determining whether the head pointer has changed looks at both the 32 bit address field and the 32 bit version field of the head pointer, and concludes a change has occurred if either one of these fields has changed.  
         [0058]    While the invention has been described in detail by specific reference to preferred embodiments, it is understood that variations and modifications thereof may be made without departing from the true spirit and scope of the invention.