Abstract:
A locking arrangement for data structures is provided that prevents deadlocks, but still allows different threads to simultaneously obtain locks on different nodes of a data structure for both read and write operations. The locking system differentiates locks based on a priority hierarchy. The locking system will fail a request to lock one or more resources in a data structure if access to those resources has already been restricted by a conflicting lock of an equal or higher priority. The locking system may also employ preemptable and non-preemptable locks such that, if a preemptable lock with a lower priority has restricted access to resources, then the locking system will preempt the lower priority lock in favor of a conflicting higher priority lock. Alternately, if a non-preemptable lock with a lower priority has restricted access to resources, then the locking system will wait until the lower priority lock is removed before implementing a requested conflicting higher priority lock. This locking arrangement allows high priority operations that require higher performance and efficiency to receive preferential access to a data structure without preventing lower priority operations from accessing the data structure, while preventing deadlocks between different operations.

Description:
[0001]    FIELD OF THE INVENTION  
           [0002]    Aspects of the present invention are directed to a locking mechanism for locking resources in a data structure, such as a tree data structure. More particularly, aspects of the present invention are directed to a locking technique that differentiates locks based on their priority, in order to avoid deadlocks.  
         BACKGROUND OF THE INVENTION  
         [0003]    Computers arrange data into organized structures, so that the data can be easily located and accessed. One type of commonly-used data structure is the tree structure. With this structure, related pieces of data form individual nodes in the tree. Each node (except for the root node) will have only a single parent node, but may have a plurality of sibling nodes and a plurality of child nodes. Conventionally, a node A is referred to as a descendant of node B if node A&#39;s parent is node B, or if node A&#39;s parent is a descendant of node B. Similarly, node A is referred to as an ancestor of node B if node B is a descendant of node A.  
           [0004]    [0004]FIG. 1 graphically illustrates how a tree structure can be used to organize information.  
           [0005]    More particularly, this figure illustrates how a tree structure can be used to organize data relating to electronic ink, so that the ink can be manipulated by a user or recognized by a recognition function of an application. Electronic ink may be made up of strokes, with each stroke corresponding to, for example, movement of a pointing device. Each stroke includes information defining the properties of the stroke, such as the data points making up the stroke, a directional vector of the stroke, a color of the stroke, and a thickness at which the stroke is to be rendered on a display.  
           [0006]    While strokes can be individually manipulated, it generally is more efficient to first organize strokes before manipulating them. Thus, a parser may be used to establish relationships between individual strokes, and then organize the strokes into larger units for editing or handwriting recognition. For example, a parser may be used to associate groups of related strokes together into units that form a word. Similarly, the parser may associate groups of one or more words together to form a line, and associate groups of one or more lines together to form a block or paragraph. The parser may then associate groups of one or more blocks or paragraphs together to form a single page or a document.  
           [0007]    A parser typically will need to analyze electronic ink several times to produce a tree structure that accurately represents the relationships between the electronic ink strokes. Moreover, each time that the electronic ink is edited, the parser will need to update the tree. The parser may therefore need to operate frequently and for prolonged periods of time. To avoid having the parser constantly interfere with active software applications each time that it needs to refine the tree structure, the parser may instead continuously operate in the background with some environments.  
           [0008]    [0008]FIG. 1 illustrates a tree structure  101  representing the results that might typically be provided by a parser. The tree  101  includes word nodes  103 . Each word node  103  contains the data for the individual strokes that make up a corresponding word W. More particularly, if the parser has determined that a group of strokes makes up a word W, then the data for those strokes are contained (or reference by) the word node  103  representing the word W.  
           [0009]    If multiple words W are associated by the parser with a single line L, then the word nodes  103  for the words W are arranged as children of a line node  105  corresponding to the line L. The line nodes  105  may include data common to all of its children, such as the color or thickness of the ink making up the words W in the line L. Line nodes  105 , corresponding to lines L that the parser has associated into a block B, are then arranged as children of a block node  107  corresponding to the block B. The block nodes  107  in turn serve as children of a page node  109 , which, in the illustrated example, is the root node for the tree  101 . Of course, if the parser recognized multiple page boundaries, then the page node  109  might itself be a child of a root node corresponding to the entire document.  
           [0010]    A number of different program threads may seek to concurrently access the information provided in the tree  101 . For example, if a user is editing the electronic ink with a notetaking application, then the notetaking application will employ a user interface thread that changes the organization of the tree  101  to correspond with the user&#39;s edits. Thus, the user interface thread will attempt to execute read or write operations on one more nodes of the tree  101 . On the other hand, the notetaking application will also employ a parser thread that may be continually refining the structure of the tree  101  in the background, as noted above. The parser thread may thus also attempt to execute a read or write operation on one or more nodes of the tree  101  at the same time as the user interface thread. Of course, other software applications may also employ threads that could concurrently attempt to access one more nodes of the tree  101  for various reasons.  
           [0011]    Moreover, even a single software thread may attempt to sequentially execute one or more read or write operations on one more nodes of the tree  101 . For example, in order to move a word W to a line L, the user interface thread may need to execute a read operation on the line node  105  corresponding to the line L, and execute a write operation on the subtree formed by the word node  103  corresponding to the word W.  
           [0012]    As will be appreciated by those of ordinary skill in the art, it would be very undesirable to allow different threads to concurrently execute conflicting read or write operations on the same node. Accordingly, a thread seeking to access a node of a data structure must first initiate a “lock” on that node, to prevent a conflicting read or write operation of another thread from being executed on that node before its own read or write operation is complete. While the use of locks prevents conflicting read or write operations from concurrently executing on the same node, it creates new problems that can potentially stop the operation of the computer.  
           [0013]    For example, referring to FIG. 2, a user interface thread may act to move a word W corresponding to the subtree  201  into the line L represented by the line node  203 , as graphically illustrated by the dotted line  205 . To complete this task, the user interface thread must request a write lock on the subtree  215 . The user interface thread would then also request a write lock on the subtree  207  (that is, the subtree that includes the line node  203 ). Similarly, the parser thread may act to move the word W corresponding to the subtree  209  into the line L represented by the line node  211 , as graphically illustrated by the dotted line  213 . In order to complete its task, the parser thread would request a write lock on the subtree  207 , and request another write lock on the subtree  215  (that is, the subtree that includes the line node  211 ).  
           [0014]    A problem arises if, for example, the user interface thread obtains a write lock on the subtree  215 , but cannot obtain a write lock on the subtree  207  before the parser thread obtains a write lock on the subtree  207 . In this situation, the user interface thread will wait for access to the subtree  207  until the parser thread&#39;s lock on the subtree  207  is lifted. The parser thread, however, will maintain its write lock on the subtree  207  until it can acquire a write lock on the subtree  215 . Because the user interface thread will maintain its lock on the subtree  215  until it can also obtain a lock on the subtree  207 , both the user interface thread and the parser thread will reach a deadlock. That is, neither the user interface thread nor the parser thread will be able to complete its task until the other finishes. This situation will effectively stop the operation of both the user thread and the parser thread, and may even impact the operation of other software applications being run by the computer.  
           [0015]    One solution to this problem is to allow a single software thread to obtain a lock on the entire data structure. Thus, the user interface thread would be able to obtain a lock on the entire tree  101 . The user interface thread could then execute read and write operations as necessary, without interference from other threads. While this solution avoids the problem of deadlocks between different threads, it reduces the performance of other operations requiring access to the data structure. That is, allowing only one thread to use the data structure at any given time unnecessarily delays the operation of other threads that need the information in the data structure. For example, if the parser thread obtains a lock to the entire tree  101  in order to access the subtree  201 , then the user interface thread may not simultaneously access the subtree  217 , even though accessing the subtree  217  would not interfere with the parser thread&#39;s access to the subtree  201 . Instead, the user interface thread must first wait for the parser thread to release the lock on the entire tree  101  before it can access the subtree  217 , which may substantially delay the operation of the user interface thread.  
           [0016]    Another solution to avoid deadlock is to allow a thread executing a write operation to obtain a lock on the entire data structure, while permitting different threads executing read operations to obtain concurrent locks. With this arrangement, a thread attempting to execute a write operation must either wait until all currently executing read operations are completed, or preempt (that is, prematurely end) the executing read operations. Thus, this solution also unnecessarily reduces the performance of operations requiring access to the data structure.  
           [0017]    In addition to avoiding unnecessary performance reduction, it may actually be desirable to allow multiple threads to concurrently execute both read and write operations on a data structure. For example, as noted above, it may be useful to have the parser thread invisibly operate as a background process, even while the user is employing the user interface thread to manipulate the electronic ink. If the parser thread cannot execute both read and write operations on the tree  101  concurrently with, for example, the user interface thread, then the parser thread may noticeably prevent or delay the user interface thread from executing write operations.  
           [0018]    It thus would be desirable to have a locking system that prevents deadlocks from occurring between different threads, but which does not unnecessarily reduce the performance of those threads. More particularly, it would be desirable to have a locking system for a data structure that allows different threads to concurrently obtain locks on different nodes of the data structure for both read and write operations.  
         SUMMARY OF THE INVENTION  
         [0019]    Advantageously, various aspects of the invention provide a locking arrangement for data structures that prevent deadlocks, but which still allows different threads to simultaneously obtain locks on different nodes of a data structure for both read and write operations. The locking system according to the invention differentiates locks based on a priority hierarchy. The locking system will fail a request to lock one or more resources in a data structure if those resources have already been locked with a non-preemptable, conflicting lock of an equal or higher priority.  
           [0020]    More particularly, if a preemptable lock with a lower priority has locked the resources, then the locking system will preempt the lower priority lock in favor of a conflicting higher priority lock. Alternately, if a non-preemptable lock with a lower priority has locked the resources, then the locking system will wait until the lower priority lock is removed before implementing a requested conflicting higher priority lock. Thus, high priority threads that require higher performance and efficiency, such as user interface threads, may receive preferential access to a data structure without preventing lower priority threads, such as a parser thread operating as a background process, from accessing the data structure. In addition, the locking technique still prevents deadlocks from occurring between different threads.  
           [0021]    These and other features and aspects of the invention will be apparent upon consideration of the following detailed description. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    [0022]FIGS. 1 and 2 show a schematic diagram of a tree data structure for organizing data relating to an electronic ink document.  
         [0023]    [0023]FIG. 3 shows a schematic diagram of a general-purpose digital computing environment that can be used to implement various aspects of the invention.  
         [0024]    [0024]FIG. 4 shows a locking system for providing access to a data structure according to an embodiment of the invention.  
         [0025]    [0025]FIG. 5 illustrates a flowchart showing a process for implementing a locking technique according to an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0026]    Introduction  
         [0027]    A locking system according to the invention differentiates locks based on a priority hierarchy. Some embodiments of the invention may also distinguish locks for two types of operations on a data structure: a write operation and a read operation. A lock for a write operation (sometimes referred to as a “write lock”) by one thread will prevent any other operation by another thread from obtaining a lock on the locked up resources. A lock for a read operation (sometimes referred to as a “read lock”) by one thread will then prevent a write lock from being obtained on the locked up resources by another thread.  
         [0028]    Thus, two concurrent write locks from different threads to the same resources will conflict with each other, as the modification of the resources by the write operation of one thread will affect the results produced by the write operation of the other thread. Likewise, a concurrent write lock and a read lock from different threads on the same resources will conflict with each other for the same reason.  
         [0029]    Two concurrent read locks, even to the same resources and from different threads, typically will not conflict. That is, because the execution of one read operation will usually not interfere with the results obtained by another read operation, then a locking system may classify all concurrent read locks, regardless of their source, as non-conflicting in order to optimize access to the data structure. If, however, a thread does employ read operations that may interfere with the read operations of another thread, then two concurrent read locks from different threads to the same resources may also be considered conflicting locks. Alternately, a locking system may forego efficiencies obtained by distinguishing read locks from write locks, and simply treat all locks as conflicting.  
         [0030]    In addition to locking the resources specified in a lock request, a lock may also restrict access in some way to other resources. For example, with a tree data structure, operations on a given node may advantageously be applied to all of that node&#39;s descendants. This frees a thread from having to obtain a separate lock each time that it accessed a different node in a subtree. Moreover, this facilitates consistently applying an operation to an entire subtree. Similarly, an operation on a node should also be respected on any of the nodes in the chain of parents leading from a locked node to the root of the entire tree. For example, if one thread executes a write operation on a child node while another thread executes a read operation on a parent node, then the results of the read operation may be invalid.  
         [0031]    Thus, with some embodiments of the invention, a lock on a node will also prevent a conflicting lock from being obtained on both ancestors of that node and descendants of that node. More particularly, for some embodiments of the invention, a lock on a specified node will also lock all of its descendants (that is, the subtree of nodes defined by taking the specified node as the root node), and prevent conflicting locks from being obtained on the ancestors of the specified node. With other embodiments of the invention, however, a lock on a specified node may simply prevent conflicting locks from being obtained on the ancestors or descendants of the specified node.  
         [0032]    By differentiating locks according to priority, the locking system of the invention will prevent a request for a lock from waiting for resources that are already locked up by a lock with an equal or higher priority. This allows different threads to concurrently access different portions of a data structure without causing a deadlock, as will be explained in detail below.  
         [0033]    Exemplary Operating Environment  
         [0034]    As will be appreciated by those of ordinary skill in the art, a locking technique according to the invention may be implemented using software. That is, a locking system according to the invention may be described in the general context of computer-executable instructions, such as program modules, executed by one or more computing devices. Generally, program modules include routines, programs, objects, components, data structures, etc. that perform particular tasks or implement particular abstract data types. Typically the functionality of the program modules may be combined or distributed as desired in various embodiments.  
         [0035]    Because the invention may be implemented using software, it may be helpful for a better understanding of the invention to briefly discuss the components and operation of a typical programmable computer on which various embodiments of the invention may be employed. FIG. 3 illustrates an example of a computing device  301  that provides a suitable operating environment in which various embodiments of the invention may be implemented. This operating environment is only one example of a suitable operating environment, however, and is not intended to suggest any limitation as to the scope of use or functionality of the invention. Other well known computing systems, environments, and/or configurations that may be suitable for use with the invention include, but are not limited to, personal computers, server computers, hand-held or laptop devices, multiprocessor systems, microprocessor-based systems, programmable consumer electronics, network PCs, minicomputers, mainframe computers, distributed computing environments that include any of the above systems or devices, and the like.  
         [0036]    The computing device  301  typically includes at least some form of computer readable media. Computer readable media can be any available media that can be accessed by the computing device  301 . By way of example, and not limitation, computer readable media may comprise computer storage media and communication media. Computer storage media includes volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of information such as computer readable instructions, data structures, program modules or other data. Computer storage media includes, but is not limited to, RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, digital versatile disks (DVD) or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, punched media, holographic storage, or any other medium which can be used to store the desired information and which can be accessed by the operating environment  301 .  
         [0037]    Communication media typically embodies computer readable instructions, data structures, program modules or other data in a modulated data signal such as a carrier wave or other transport mechanism, and includes any information delivery media. The term “modulated data signal” means a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media includes wired media such as a wired network or direct-wired connection, and wireless media such as acoustic, RF, infrared and other wireless media. Combinations of any of the above should also be included within the scope of computer readable media.  
         [0038]    With reference to FIG. 3, in its most basic configuration the computing device  301  typically includes a processing unit  303  and system memory  305 . Depending on the exact configuration and type of computing device  301 , the system memory  305  may include volatile memory  307  (such as RAM), non-volatile memory  309  (such as ROM, flash memory, etc.), or some combination of the two memory types. Additionally, device  301  may also have mass storage devices, such as a removable storage device  311 , a non-removable storage device  313 , or some combination of two storage device types. The mass storage devices can be any device that can retrieve stored information, such as magnetic or optical disks or tape, punched media, or holographic storage. As will be appreciated by those of ordinary skill in the art, the system memory  305  and mass storage devices  311  and  313  are examples of computer storage media.  
         [0039]    The device  301  will typically have one or more input devices  315  as well, such as a keyboard, microphone, scanner or pointing device, for receiving input from a user. The device  301  will typically also have one or more output devices  317  for outputting data to a user, such as a display, a speaker, printer or a tactile feedback device. Other components of the device  301  may include communication connections  319  to other devices, computers, networks, servers, etc. using either wired or wireless media. As will be appreciated by those of ordinary skill in the art, the communication connections  319  are examples of communication media. All of these devices and connections are well know in the art and thus will not be discussed at length here.  
         [0040]    A Data Structure System  
         [0041]    [0041]FIG. 4 illustrates a data structure system  401  according to one embodiment of the invention. As shown in this figure, the data structure system  401  communicates with one or more threads  403 - 407 . More particularly, the threads  403 - 407  request access to information resources maintained by the data structure system  401 . In the illustrated embodiment, each of the threads  403 - 407  is generated by the same software application, but two or more of the threads  403 - 407  may alternately be generated by different software applications.  
         [0042]    The data structure module  409  maintains information in the data structure  411 . The data may be any type of information such as, for example, data relating to an electronic ink document. It should be noted that, while FIG. 3 schematically illustrates the data structure  411  as a tree structure, the data structure module  409  may also maintain data in an alternate structure of any desired typed or configuration. The data may physically be stored in the system memory  305 , the removable storage  311 , the non-removable storage  313  or a combination thereof using, for example, any suitable database software application.  
         [0043]    The data structure system  401  also includes a lock request evaluation module  413 . The lock request evaluation module  413  receives requests to access one or more resources of the data structure  405  from the threads  403 - 407 . Typically, a request to access resources will identify the node (or nodes) for which access is requested (sometimes referred to hereafter as the “requested node”), and the type of access requested (that is, whether the thread will access the requested node with a read operation or a write operation). The access request will also include a request to lock the requested node, along with a priority for the requested lock. In addition, the access request may specify whether the requested lock will be a preemptable lock or a non-preemptable lock.  
         [0044]    In response to receiving a lock request, the lock request evaluation module  413  determines whether the lock request will succeed or fail. If the lock request evaluation module  413  decides to approve a requested lock, it then passes the lock request to the lock maintenance module  415 . The lock maintenance module  415  tracks existing locks. Thus, when the requested node becomes available, the lock maintenance module  415  will initiate the requested lock so that the thread can obtain the specified access to the requested node. The lock maintenance module  415  will then keep track of the new lock as well.  
         [0045]    Operation of the Data Structure System  
         [0046]    The operation of the lock request evaluation module  413  and the lock maintenance module  415  will now be discussed in more detail with reference to the flowchart illustrated in FIG. 5. In step  501 , a thread  403 ,  405  or  407  submits a request to access one or more resources (for example, to access to a subtree) in the data structure  409 . The access request identifies the resources for which access is requested, and the type of access requested. That is, the access request will specify whether the access is to execute a read operation or a write operation. It will also request a lock on the root node of the subtree, along with a priority for the lock.  
         [0047]    Upon receiving a lock request, the lock request evaluation module  413  first determines if the requested lock is a write lock. If the requested lock is not a write lock (that is, if the requested lock is a read lock), then in step  505  the lock request evaluation module  413  determines if access to the requested node has been restricted by a conflicting lock. That is, the lock request evaluation module  413  determines if there is an existing write lock on the requested node. The lock request evaluation module  413  also determines if there are any conflicting write locks on any of the ancestors or descendants of the requested node that would prevent a write lock from being obtained on the requested node. As previously noted, a write operation on a node by one thread may also affect the results of a read or write operation on an ancestor or descendant of that node by another thread. Accordingly, while the ancestors or descendants of the node may not be identified in the read lock request, the lock request evaluation module  413  also determines if a conflicting write lock has already been obtained for these resources. Thus, the lock request evaluation module  413  determines if the requested lock will conflict with an existing lock that would restrict access to any of the requested resources.  
         [0048]    If none of the requested node, its ancestors and its descendants have been locked up by a conflicting lock, then the lock request evaluation module  413  immediately approves the requested lock in step  507 , and passes the approved lock request onto the lock maintenance module  415 . If the requested node, one of its ancestors or one of its descendant has already been locked by a conflicting write lock, however, then the lock request evaluation module  413  determines if the priority of the requested read lock is a high priority in step  509 . With the illustrated embodiment of the invention, the lock request evaluation module  413  recognizes only two priorities of locks, high and low. Accordingly, if the priority of the requested read lock is not high, it must be low, and thus equal to or lower than the priority of the conflicting write lock on the requested lock, its ancestor or descendant. As a result, the lock request evaluation module  413  fails the requested lock in step  511 .  
         [0049]    If, however, the requested read lock has a high priority, then in step  513  the lock request evaluation module  413  checks to confirm that all of the conflicting write locks on the requested node, its ancestors and its descendants are low priority. If one of these conflicting write locks are high priority, then again the requested read lock is equal to this conflicting high priority write lock, and the requested read lock is failed in step  511 . If all of the conflicting write locks on the requested node, its ancestors and its descendants are low priority (and thus lower in priority than the requested read lock), then in step  515  the lock request evaluation module  413  will approve the requested read lock. In step  517 , the lock request evaluation module  413  passes the requested read lock onto the lock maintenance module  415 , which notes that the requested read lock is waiting for the existing conflicting write locks to complete and should be implemented when these locks are completed.  
         [0050]    Returning now to step  503 , if the lock request evaluation module  413  determines that a thread has requested a write lock (that is, that the requested lock will conflict with any existing lock from another thread), then in step  519  the lock request evaluation module  413  determines if there are any conflicting non-preemptable read locks or write locks that would restrict access to the requested node. That is, the lock request evaluation module  413  determines if there is an existing conflicting lock on the requested node. It also determines if there are any existing, conflicting non-preemptable read locks or write locks on the ancestors or descendants of the requested node. If there are not (that is, if there are no existing locks or if the only existing locks are preemptable), then in step  521  the lock request evaluation module  413  voids any existing preemptable read locks on the requested nodes, its ancestors and its descendants. Then, in step  507 , it approves the requested write lock and passes the requested write lock onto the lock maintenance module  415  to be implemented.  
         [0051]    If, however, there is one or more conflicting non-preemptable read locks or write locks on a requested node, one of its ancestors or one of its descendants, then in step  523  the lock request evaluation module  413  determines if any of these conflicting locks has a high priority. Again, because the lock request evaluation module  413  in this embodiment only recognizes two priorities, if any of these conflicting locks has a high priority, then the priority of the requested lock must be equal to or lower than the priority of these conflicting locks. Thus, in step  511 , the lock request evaluation module  413  fails the requested write lock.  
         [0052]    On the other hand, if none of the conflicting locks on the requested node, its ancestors or its descendants has a high priority, then in step  525  the lock request evaluation module  413  determines the priority of the requested write lock. If it is low, then again it must be equal to the priority of the conflicting locks, and is failed in step  511 . If, however, the priority of the requested write lock is high, it is greater than the priority of any conflicting lock on the requested node, its ancestors and its descendants, and in step  515  the lock request evaluation module  413  will approve the requested read lock. In step  517 , the lock request evaluation module  413  passes the requested read lock onto the lock maintenance module  415  to be implemented when the existing conflicting write locks are completed.  
         [0053]    In the illustrated embodiment, the locking system uses only two priorities. It should be noted, however, that other embodiments of the invention may employ a priority hierarchy with any number of desired priorities. As in the embodiment described above, with these alternate embodiments of the invention a requested lock will not wait on a conflicting, non-preemptable lock of equal or higher priority. For example, if the locking system according to the invention employed three priorities, high, medium and low, then a lock request for a medium priority lock would not wait for an existing conflicting lock with a high or medium priority to complete, but would wait for an existing conflicting lock with a low priority to complete. Similarly, a lock request for a high priority lock would not wait for an existing conflicting lock with a high priority to complete, but would wait for an existing conflicting lock with a medium or low priority to complete. Of course, the implementation of still greater numbers of different priorities will be apparent from the foregoing description.  
         [0054]    In the foregoing illustrated embodiment, any write lock will preempt a preemptable lock, regardless of the relative priority of the different locks. It should be appreciated, however, that alternate embodiments of the invention may only allow a write lock to preempt a preemptable read lock of lower priority. Also, it should be noted that, to facilitate an understanding of the invention, the invention has been explained above with particular emphasis on prioritizing locks between different threads of a single software application. As will be appreciated by those of ordinary skill in the art from the foregoing description, however, the invention may also be employed to prevent lock conflicts between threads of different software applications. Still further, while the above discussion of the invention distinguishes locks for read operations from locks for write operations, various embodiments of the invention need not make that distinction. Instead, as previously noted, these embodiments of the invention may characterize all locks from different threads as conflicting locks.  
       CONCLUSION  
       [0055]    Although the invention has been defined using the appended claims, these claims are exemplary in that the invention may be intended to include the elements and steps described herein in any combination or sub combination. Accordingly, there are any number of alternative combinations for defining the invention, which incorporate one or more elements from the specification, including the description, claims, and drawings, in various combinations or sub combinations. It will be apparent to those skilled in the relevant technology, in light of the present specification, that alternate combinations of aspects of the invention, either alone or in combination with one or more elements or steps defined herein, may be utilized as modifications or alterations of the invention or as part of the invention. It may be intended that the written description of the invention contained herein covers all such modifications and alterations. For instance, in various embodiments, a certain order to the data has been shown. However, any reordering of the data is encompassed by the present invention.