Abstract:
An ordered semaphore management subsystem and method for use in an application system which includes a plurality of processors competing for shared resources each of which is controlled by a unique semaphore. The subsystem generates an ordered semaphore field (OSF) corresponding to each processor in a linked list of processors and assigns one of four statuses to the OSF depending on the position the processor occupies in the linked list of processors competing for the shared resources. The four states are (1) semaphore head (SH); (2) behind semaphore head (BSH); (3) semaphore head behind (SHB); and (4) skip (Skip). Only the SH processor is allocated the semaphore when requested. A processor not in the SH state will be denied the semaphore even if is available to assure sequential access.

Description:
[0001]    This application claims the benefit of the filing date of provisional application Serial No. 60/301,598, filed Jun. 27, 2001. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The invention relates to semaphore management subsystems and more particularly to generic ordered semaphore management subsystems and a method for managing ordered semaphores suitable for use with any multi-threaded (multiprocessor) system requiring in part or in hole in line or sequential processing of a task divided and distributed among a plurality of threads or processors for later recombination.  
         BACKGROUND  
         [0003]    While the invention is generic in nature and capable of use with a large variety of multi-threaded processor systems, it will be described in conjunction with a multi-threaded processor system such as the IBM Part No. IBM32NPR161EPXCAE133 Network Processor which employs a plurality of processors or threads each of which concurrently process data frames which may be from the same or different data flows. The individual threads/processors share common resources in the network processor. Semaphores defined to be associated with specific resources are used to allocate the specific resources to the individual threads as requested.  
           [0004]    Within such a network processor several data frames are processed at the same time. Each data frame is processed by one processor/thread. Each processor/thread operates independently from all the other processors/threads. Thus, as the software (picocode) processes a data frame, the software has no knowledge of other frames which have been, are being, or will be processed. As data frames are processed, a thread may need access to a shared resource. This shared resource is shared among all threads. To allow a thread access to the resource without interference from other threads, semaphores are used. A semaphore is a mechanism which allows a processor/thread to use a resource without interference from another processor/thread. Semaphores exist in almost every multi-processor environment where multiple processors can access common resources. A semaphore is used to ensure that one and only one processor/thread has “ownership” or use of a given resource at any given time.  
           [0005]    A network processor is a multi-processor environment with resources which can be accessed by all processors/threads. Thus, semaphores are an intricate part of network processors. A discussed above, network processors process data frames which belong to one or more data flows. Traditionally, semaphores are implemented in software using “read modify write” or “test and set” instructions. When these instructions are used as a basis to create and allocate semaphores, valuable system resources must be used. To implement a semaphore, system memory must be used. To access a semaphore, several lines of code must be executed. If these system resources were not used for semaphore implementation, they could be used for other functions or provide a performance increase by not executing extra line(s) of code.  
           [0006]    When semaphores are implemented in software, several lines of code must be executed to access and lock the semaphore, impacting performance. If the semaphore is unavailable (locked by another thread/processor), the software would need to poll on the semaphore. This would waste valuable bandwidth on the arbitrated memory holding semaphore locks to be accessed by all threads/processors. To implement a fair semaphore access in software requires more system memory and lines of code. For example, if a semaphore is locked, the thread/processor would need to put itself in a queue waiting for access. This queue would be implemented in system memory and require software management impacting performance. This allows threads/processors to have fair access to resources.  
           [0007]    In a software semaphore environment, multiple threads/processors cannot unlock their respective semaphores at the same time. Typically, all the semaphores are in the same system memory. Each thread/processor must arbitrate to access the memory to unlock their semaphore. This may add to the processing time of other threads/processors waiting to access the same memory to access the semaphore locks. The same is true for locking semaphores. When semaphores are implemented in software, only one semaphore can be unlocked/locked at a time since all the semaphores reside in a common area of system memory.  
           [0008]    In the IBM Network Processor System identified above a device termed Completion Unit monitors the order in which frames or packets in a flow are processed by the threads or Dyadic Protocol Processor Units (DPPUs) and generates information used by a semaphore sub-system to control the order in which semaphores are assigned. Such systems require ordered semaphores which must perform two functions. First, the well known semaphore function, ensure that one and only one processor/thread has access to a single resource at any time. And second, ordered semaphores must ensure that the processors/threads which are processing data frames of the same data flow access the common resource in frame order, for example, an e-mail message which must be encrypted using an encryption co-processor shared among all of the processors/threads. The encryption of the data frames must occur in order to properly encrypt the message. The software would use an ordered semaphore mechanism to access the encryption co-processor.  
           [0009]    This would ensure two things. First, only one processor/thread accesses the co-processor at a time. And second, the encryption of the data frames of the data flow (e-mail message) occurs in order. Ordered Semaphores are needed since processing time for each data frame can be different. Data frames from the same data flow will take different amounts of time to process. For example, tree searches for each data frame can take different amounts of time. Threads which share a common ALU will stall occasionally to allow the other thread to process data. Thus, frames in the same data flow being processed by different threads will attempt to access a shared resource at different times and not in data flow order. Because of this, ordered semaphores are required to ensure the shared resource is accessed in data flow order.  
           [0010]    The Completion Unit logic block contains all the information required to put processed data frames (received from processors/threads) back in the correct order for each data flow. U.S. applications Ser. No. 09/479,028 filed Jan. 7, 2000 and Ser. No. 09/548,906 filed Apr. 13, 2000 incorporated herein by reference describe how the Completion Unit performs this function. Within the completion unit, linked lists of the data frames assigned to processors/threads represent the data frame order of the data flows. One linked list exists for each data flow which currently has a data frame being processed by a processor/thread. The head of the linked list is associated with a processor/thread. It is from this processor/thread that the next processed data frame is to be taken from and sent out onto the network. When the processed data frame is sent, the head of the linked list is removed and the next element of the linked list is examined; see the referenced applications for details.  
         SUMMARY OF THE INVENTION  
         [0011]    The invention contemplates an ordered semaphore management subsystem and method for use in an application system which includes a plurality of shared resources each of which is controlled by a unique semaphore; a plurality of processors adapted to perform similar tasks using one or more shared resources controlled in a sequential manner by the semaphores on assigned segments of a continuous data stream; a linked list of processor labels each identifying a processor and defining the order in which the processed data is to be assembled. Each label includes, a data flow ID, a data flow head field, a data flow tail field, and a pointer field pointing to the next label in the linked list.  
           [0012]    The subsystem includes a logic circuit responsive to the creation of a label included in the linked list for generating and encoding a predetermined state in an ordered semaphore field (OSF) corresponding to the created label. The states include:  
           [0013]    a semaphore head (SH) state,  
           [0014]    a behind semaphore head (BSH) state,  
           [0015]    a semaphore head behind (SHB) state and,  
           [0016]    a skip (Skip) state.  
           [0017]    A second logic circuit responsive to a request for a semaphore from a processor examines the encoded ordered semaphore field corresponding to the processor&#39;s label and grants the semaphore request when the field is in the SH state. A third logic circuit responsive to a change in the status of a processor updates the ordered semaphore field corresponding to the label associated with the processor. It updates the OSF corresponding to the label identified by the pointer in the label of the processor whose status changed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is a block diagram of an application system which incorporates a semaphore manager subsystem according to the invention;  
         [0019]    [0019]FIG. 2 is a block diagram of a semaphore manager subsystem according to the invention;  
         [0020]    [0020]FIG. 3 is a diagram of the semaphore value storage  21  of FIG. 2;  
         [0021]    [0021]FIGS. 4A and 4B are flow diagrams defining the lock command logic  22  of FIG. 2;  
         [0022]    [0022]FIG. 5 is a flow diagram defining the semaphore unlock command logic  23  of FIG. 2;  
         [0023]    [0023]FIG. 6 is a flow diagram defining the error and exit logic  24  of FIG. 2;  
         [0024]    [0024]FIG. 7 is a flow diagram the reservation release command logic  25  of FIG. 2;  
         [0025]    [0025]FIG. 8 is a diagram of a linked list thread element including an ordered semaphore field OSF according to the invention; and, FIGS.  9 - 12  are flow diagrams of the completion unit interface logic. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]    As pointed out above the completion unit of the IBM network processor system contains all the information required to put processed data frames (received from processors/threads) back in the correct order for each data flow. By adding a two bit Ordered Semaphore Field (OSF) to each element in the linked lists of the completion unit, ordered semaphore management can be achieved. The OSF allows the Semaphore Manager/Co-processor logic block to walk the linked list(s) within the completion unit before the processed data frames are sent out onto the network. The lists can reside in the completion unit or alternatively updated copies of the lists can be provided by the completion unit to the semaphore manager subsystem.  
         [0027]    The OSF added to the linked list elements represent one of four states: (1) this element is currently the Semaphore Head (SH), (2) this element is currently behind the Semaphore Head (BSH), (3) the SH is currently behind this element (SHB), and (4) this element has been removed from the linked list (Skip), with respect only to ordered semaphores.  
         [0028]    When a new element (label/thread number) is loaded into the completion unit (due to a dispatch), the Ordered Semaphore Field (OSF) will be set to one of three states. The OSF can be set to indicate the element has been removed from the linked list (with respect to ordered semaphores). This is used in the case where ordered semaphores are disabled or it is known that the software will not use ordered semaphores to process the dispatched frame. If the label/thread being loaded into the completion unit is linked into an existing linked list behind an existing Semaphore Head (SH) field, the OSF is marked as BSH. If the label/thread being loaded into the completion unit is the start of a new linked list, or is being linked into an existing linked list which does not have a OSF =SH, the OSF is marked SH.  
         [0029]    When an element is SH, this allows the Semaphore Manager/Co-processor to grant an ordered semaphore for the thread to access the protected resource. When an element is in the BSH state, the Semaphore Manager/Co-processor must wait to grant the thread access to the ordered semaphore protected resource even if the resource is available. At any time, a thread can indicate to the Semaphore Manager/Co-processor (using the reservation release command) that it no longer needs the ordered semaphore access. If this occurs when the thread is the SH, the two bit field is changed to indicate this element is currently SHB, and the next element which has not been removed from the linked list (with respect to ordered semaphores) is marked as the SH.  
         [0030]    If the Semaphore Manager/Co-processor, in response to the reservation release command, indicates that a particular thread no longer needs the ordered semaphore access and the thread is currently BSH, the element will be marked Skip and removed from the linked list with respect to ordered semaphores. When the semaphore head reaches this thread, the semaphore head will simply skip to the next element (thread) in the list. By adding additional Ordered Semaphore Fields, more than one ordered semaphore can be supported per data flow. For example, consider adding two Ordered Semaphore Fields to each element within the completion unit. The software can be designed to access two different resources, or the same resource twice, using Ordered Semaphores. Again, the additional logic to the completion unit interface logic block is minimal if the function is included in the completion unit logic block.  
         [0031]    The Semaphore ManageilCo-processor does not add or remove labels/thread numbers from the linked lists within the completion unit. The Semaphore Manager/Co-processor can mark labels/thread numbers as Skip. This allows the thread to continue processing without having to access the ordered semaphore. The “Reservation Release” command of the Semaphore Coprocessor/Manager can mark a label/thread as “Skip”. When the thread in front releases the ordered semaphore, the ordered semaphore head will pass to the thread behind the thread marked “Skip”. The Semaphore Manager/Co-processor is simply walking the pre-existing linked lists within the completion unit (if they are included in the completion unit logic block) to achieve ordered semaphore operation. The OSF can be used to mark the associated label/thread number as “removed” from the linked list with respect to ordered semaphores by setting the OSF to Skip. The label/thread numbers remain in linked lists to support the enqueue of processed data frames. The “Reservation Release” or “skip” marking is non-blocking on every thread. Thus, threads performing “Reservation Release” commands do not wait to become the head of the ordered semaphore queue before completing the instruction. This increases system performance by reducing processing time.  
         [0032]    In FIG. 1 a  network processor such as the IBM processor identified above includes an input/output data storage unit  11  which stores a plurality or stream of data frames which require processing. A dispatching unit  12  transfers individual data frames to a plurality of processors  13 - 1 - 13 - n  which process the individual frames received from the dispatching unit  12 . The processors  13 - 1 - 13 - n  pass the processed data frames on to a completion unit  14  which reassembles the data frames before passing the reassembled stream of data frames on to a second input/output unit  15 .  
         [0033]    Each of the processors  13 - 1 - 13 - n  includes a semaphore coprocessor  13   p  which interfaces a hardware semaphore manager subsystem  16  constructed according to the invention. The semaphore subsystem  16  is implemented in hardware and interfaces with, for example, the Dyatic Protocol Processor Unit (DPPU) of the using processing system. Each DPPU contains four threads which can each process one data frame. Each DPPU has one Semaphore Co-Processor associated with it. The threads in a given DPPU interface to the one Semaphore Co-Processor within the DPPU. The multiple Semaphore Co-Processors all communicate with the central Semaphore Manager subsystem. The Semaphore Manager subsystem  16  contains all of the tables and control logic to lock, unlock, and arbitrate for semaphores. The semaphore manager  16  communicates with the completion unit  14  over a bus  17 .  
         [0034]    Only one processor instruction is executed to lock a semaphore, and only one processor instruction is executed to unlock a semaphore. This saves instruction memory and processing time when compared to software implementations of semaphores. An unlock instruction from a thread executes immediately. Nothing can block unlock instructions within the Semaphore Manager subsystem, including other unlock instructions from other threads. When semaphores are implemented in software, only one semaphore can be unlocked at a time since all the semaphores reside in a common area of system memory.  
         [0035]    Fairness algorithms can be implemented in the Hardware Semaphore Manager subsystem. This provides fairness of access to a semaphore when multiple threads/processors request access to the same semaphore. This eliminates the need for software and system memory to be utilized to implement fairness algorithms. This increases performance by reducing frame processing time and instruction memory utilization, and does not use any system memory.  
         [0036]    The Hardware Semaphore Manager subsystem contains the semaphores. Each semaphore is an N bit value. For example, each semaphore could be 32 bits. This allow 2^ 32 resources to be managed by the Semaphore Manager. Since the Semaphore Manager subsystem is generic and designed to be used by a wide variety of systems, it is the responsibility of the using system programmer to attach a meaning to a semaphore. That is, the Semaphore Manager does not know what a Semaphore represents. It is just a string of 32 bits. Semaphores can be seen as having 32-bit address space and the programmer can map this to anything, like the Tree Search Memory, the Data Store, the Embedded PowerPC, etc. There are two advantages here. First, no system memory is used for semaphore management. Second, the Hardware Semaphore Manager implementation is generic with respect to the number of and types of system resources which must be managed by semaphores. As the Network Processor evolves and shared resources are added or removed, the Semaphore Manager does not need to change. The Hardware Semaphore Co-processor and Manager does not use any system memory. This eliminates the need for memory accesses, and allows the memory bandwidth to be used for other functions.  
         [0037]    In FIG. 2 the semaphore coprocessors  13   p - 1 - 13   p - n  communicate with the Semaphore Manager subsystem  16  via a bus  20 . The subsystem  16  includes a semaphore value storage  21  (FIG. 3), semaphore lock command logic  22  (FIGS. 4A and 4B), semaphore unlock command logic  23  (FIG. 5) and semaphore exit and error detection logic  24  (FIG. 6). It also includes reservation release command logic  25  (FIG. 7) and completion unit interface logic  26  (FIGS.  9 - 12 ).  
         [0038]    [0038]FIG. 3 is a block diagram of the semaphore value storage. The storage can be based upon a RAM, CAM, or discrete latches. For each thread there exists exactly two registers. The first register (Semaphore_Value) is for example a 32 bit register which holds the 32 bit Semaphore Value (Sem_Val) that can be locked by the associated thread. The second register (Semaphore_Lock) is a 1 bit register which indicates if the Sem_Val stored in the associated Semaphore_Value register is locked or unlocked. When the Semaphore_Lock register is set to 1’b, the Sem_Val in the Semaphore_Value register is locked. When the Semaphore_Lock register is reset to ‘0’b, no Sem_Val is locked by the associated software thread.  
         [0039]    A semaphore can be locked when a software thread issues a single command “Semaphore Lock” (Sem_Lock) with two parameters. The first parameter is the “Semaphore Value” (Sem_Val). This is, for example, a 32 bit value which the thread wishes to lock. The second parameter is the “Timeout Enable” (Timeout_Enable) bit. When the Timeout Enable bit is set and the requested semaphore is already locked by a different thread, the Semaphore Lock command will terminate without locking the semaphore.  
         [0040]    In the preferred embodiment, each thread has an assigned register in the semaphore value storage and is thus identified as the source of the semaphore value requested. Alternatively, the requested semaphore value could be placed in any available register along with a thread or flow ID.  
         [0041]    [0041]FIGS. 4A and 4B illustrate the function of the lock command logic. Directional blocks A, B, C and D indicate the connections between the two figures. The process starts when a thread N issues a semaphore lock command which includes a 32 bit semaphore value and a 1 bit time out enable value  400 . If the request is an ordered semaphore request and thread N is not enabled for ordered semaphores  400 -A, a queue not enabled error is generated  401  and the request is changed to an unordered request  402 . If it is not the former, it is examined to determine if it is an unordered request or an ordered request at the head of a queue  403 . If the request is either of the above it enters a round robin  404  for selection and exits FIG. 4A at A. If the answer in block  403  is “no” (in which case it is an ordered request which is not the head of a queue) it is examined to determine if the timeout enable bit is set. If the timeout enable bit is not set  405  the request loops back to  403 . If the bit is set it exits FIG. 4A at D.  
         [0042]    The non-winning threads at block  406  loop back to block  404  while the semaphore request from the winning thread is examined  407  in the semaphore value storage to determine if that semaphore value is currently locked. If the semaphore value is not locked it is examined in block  408  to determine if the request is ordered. If the request is ordered, the thread is removed from the ordered queue by sending a Pop signal  409  and the semaphore value requested is locked  410  for thread N. If the request was not ordered the requested semaphore value is locked for thread N  410  and in either case the operation for thread N is complete  411 .  
         [0043]    If the locked semaphore value requested by thread N is already locked by thread N  412 , a lock same semaphore value error for thread N is generated  413  and the operation completes at  411 . If the requested semaphore value  412  was not locked by thread N the timeout bit is examined  414  and if it is not set it loops back to  404  via C. If it is set a signal is generated for thread N indicating that the requested semaphore was not available  415  and the operation completes  411 .  
         [0044]    When a thread completes an operation with a resource controlled by a semaphore it sends a semaphore unlock command to the semaphore manager, FIG. 5 at  501 . The semaphore manager unlocks the semaphore by resetting the semaphore lock bit in the memory location associated with thread N in the semaphore memory  21 ,  502  and completes the operation for thread N  503 .  
         [0045]    When thread N exits (processing is complete for the data frame)  601  FIG. 6 the semaphore manager examines the address in memory  21  associated with thread N to determine if the thread has a locked semaphore. If a semaphore is locked  602  the semaphore manager unlocks the semaphore  603  and generates a semaphore locked at exit error for thread N  604 . If a semaphore is not locked by thread N the operation is complete.  
         [0046]    When the software on thread N issues a reservation release command  701  FIG. 7 the semaphore manager determines if the thread is enabled for ordered semaphores  702 . If it is, the semaphore manager sends a Pop signal (this signal indicates an end of ordered semaphore operation for thread N) to the completion unit interface logic  703  and completes the current operation for thread N  704 . If thread N is not enabled for ordered semaphores  702  the semaphore manager completes the current operation for thread N  704 .  
         [0047]    The completion unit in the referenced IBM network processor employs a linked list(s) of labels for managing the reordering of data frames processed by the different threads. The label illustrated in FIG. 8 is similar (with one exception discussed below) to the label used in the IBM system. The label element  800  illustrated in FIG. 8 contains the label associated with the data flow being processed and a plurality of fields. A one bit field  802  when set indicates that the thread is the head of the enqueued data flow. A one bit field  803  when set indicates that the label is valid. A one bit field  804  when set indicates that the thread is the tail of the data flow and a multi-bit field  805  is a pointer to the next label in the linked list of labels.  
         [0048]    Applicants have modified the label of the referenced IBM network management system by adding a two bit ordered semaphore field (OSF)  806  to the label. This field defines one of four states (00, 01, 10 &amp; 11) that a thread enabled for ordered semaphores can assume. A first state SH (which can be occupied by only one thread in a linked list of  800 &#39;s), indicates that the thread is currently the semaphore head. A second state BSH indicates that the thread is currently behind the semaphore head. A third state, SHB indicates that the SH is currently behind the thread. And the fourth state Skip indicates that the thread has been removed (but only with respect to ordered semaphores) from the linked list. How these states are applied and managed will become apparent from the description of FIGS.  9 - 12  below.  
         [0049]    The semaphore manager needs access to the linked list(s) of labels. In those instances where the application system allows access, the logic necessary to manage the OSF (which is the case with the IBM network system) can be achieved through the completion unit interface logic  26 . If access is not available then the application system needs to make updated copies of the linked list(s) available to the interface logic  26 . The location of the linked list(s) is not critical to the invention and in the descriptions which follow it will be assumed that the application system allows the semaphore manager access to the linked list(s) via the interface  26 .  
         [0050]    The flow diagram of FIG. 9 defines the logic for initializing an ordered semaphore field OSF. When a label is written into the label field ( 801 ), update of the OSF starts ( 901 ). The logic determines if ordered semaphores are enabled for the frame associated with the label  902 . The label is checked  903  to determine if the enqueue data flow head was set to one when the label was written. If it was the associated OSF is set to the head state SH  904  and the operation is complete. If it was not set to one and the OSF of the old tail is SHB  905 , the OSF of the label is set to SH  904 . If  905  is negative, the OSF of the label is set to BSH  906 .  
         [0051]    If block  902  is negative, block  907  determines if the enqueue data flow head was set to one when the label was written. If it was, the OSF is set to SHB  908 . If block  907  is negative, block  909  determines if the OSF of the old tail was SHB and sets the OSF of the label to SHB  908 . If block  909  is negative the label OSF is set to Skip  910 .  
         [0052]    When a Pop command (FIG. 7, 703 or FIG. 4B, 409) is received at the interface  26  FIG. 10 at block  1001 , the OSF for the thread is checked  1002  to determine if the OSF was set to SH. If it is set to SH, the field is changed to SHB state  1003 . The tail bit is checked  1004 . If the tail bit is one the process ends. If the tail bit is not one, an update semaphore head command is sent to the next element or thread in the linked list  1005  and the process ends. If block  1002  is negative, block  1006  checks the OSF to determine if it is set to the BSH. If it is, the OSF of the element or thread is set to the Skip state  1007  and the process ends. When block  1006  is negative the process ends and no change to the OSF is made.  
         [0053]    In FIG. 11, in response to the receipt of an update semaphore head command (FIG. 10, 1005 or FIG. 11, 1106) from a previous label or thread  1101 , the logic  1102  examines the OSF of the current label. If the field is BSH the field is changed to SH  1103 . If it is not BSH, it is set to SHB state  1104 . The tail bit is checked  1105 . If the tail bit is set the process ends. If the tail bit is not set an update semaphore head command is sent to the label or thread identified in the pointer  1106  and the process ends.  
         [0054]    When the completion unit receives a data flow enqueue command from a thread FIG. 12, 1201 the OSF is examined  1202 . If it is neither SH or BSH the process ends. If the field is SH or BSH, the semaphore manager generates an enqueue without Pop of ordered semaphore error message  1203 . This indicates to the software that a thread was enabled to use ordered semaphores but did not.  
         [0055]    The foregoing is illustrative of the present invention and is not to be construed as limiting the invention. While several embodiments of this invention have been described in detail, those skilled in this art will readily appreciate that many modifications are possible without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined by the claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and structural functional equivalents thereof. Therefore, it is to be understood that the foregoing is illustrative of the present invention and is not to be construed as limited to the specific embodiments disclosed, and that modifications to the disclosed embodiments, as well as other embodiments, are intended to be included within the scope of the claims appended hereto.