Locking and unlocking mechanism for controlling concurrent access to objects

• US 6,247,025 B1
• Filed: 09/22/1997
• Issued: 06/12/2001
• Est. Priority Date: 07/17/1997
• Status: Expired due to Term

• Current Assignee: International Business Machines Corporation
• Original Assignee: International Business Machines Corporation
• Inventors: Bacon, David Francis
• Primary Examiner(s): Amsbury, Wayne
• Assistant Examiner(s): HAVAN, THU THAO
• Application Number:
  US08/937,311
• Time in Patent Office: 1,359 Days
• Field of Search: 707/206, 707/103, 707/102, 707/200, 707/1, 395/683, 395/726, 395/670, 395/672, 395/877, 395/872, 711/152, 711/113, 711/170, 711/141, 711/169, 364/228
• US Class Current: 1/1
• CPC Class Codes: G06F 16/2343 Locking methods, e.g. distributed locking or locking implementation details
• CPC Class Codes: Y10S 707/99943 Generating database or data structure, e.g. via user interface
• CPC Class Codes: Y10S 707/99944 Object-oriented database structure
• CPC Class Codes: Y10S 707/99957 Garbage collection

Abstract

A lock/unlock mechanism to control concurrent access to objects in a multi-threaded computer processing system comprises two parts: a thread pointer (or thread identifier), and a one-bit flag called a “Bacon bit”. Preferably, when an object is not locked (i.e., no thread has been granted access to the object), the thread identifier and Bacon bit are set to 0. When an object is locked by a particular thread (i.e., the thread has been granted access to the object), the thread identifier is set to a value that identifies the particular thread; if no other threads are waiting to lock the object, the Bacon bit is set to 0; however, if other threads are waiting to lock the object, the Bacon bit is set to ‘1’, which indicates the there is a queue of waiting threads associated with the object. To lock an object, a single CompareAndSwap operation is preferably used, much like with spin-locks; if the lock is already held by another thread, enqueueing is handled in out-of-line code. To unlock an object, in the normal case, a single CompareAndSwap operation may be used. This single operation atomically tests that the current thread owns the lock, and that no other threads are waiting for the object (i.e., the Bacon bit is ‘0’). A global lock is preferably used to change the Bacon bit of the lock. This provides an lock/unlock mechanism which combines many of the desirable features of both spin locking and queued locking, and can be used as the basis for a very fast implementation of the synchronization facilities of the Java language.

Claims

1. A method for controlling concurrent access to an object in a multi-threaded computer processing system, the method comprising the steps of:

storing in memory a lock that is associated with said object, said lock comprising a thread identifier field and a flag field;

wherein, when said object is not locked by any thread,

a portion of said lock is set to a predetermined value that indicates that the object is not locked by any thread;

wherein, when an object is locked by a particular thread,

said thread identifier field is set to a value that identifies the particular thread,

if no other threads are waiting to lock the object, said flag field is set to a predetermined value that indicates that no other threads are waiting to lock the object, and

if other threads are waiting to lock the object, said flag field is set to a predetermined value which indicates that there is a non-empty queue of waiting threads associated with the object.

2. The method of claim 1, wherein, when said object is not locked by any thread, said portion of said lock that is set to a predetermined value that indicates that the object is not locked by any thread comprises said thread identifier field and flag field of said lock.

3. The method of claim 1, wherein, when said object is not locked by any thread, said portion of said lock that is set to a predetermined value that indicates that the object is not locked by any thread comprises said thread identifier field of said lock.

4. The method of claim 1, wherein said flag field is a single bit.

5. The method of claim 1, wherein said thread identifier field and said flag field comprise a word having a more significant portion distinct from a less significant portion, wherein said flag field is stored in said less significant portion and said thread identifier field is stored in said more significant portion.

6. The method of claim 1, further comprising the step of evaluating said lock associated with said object in order to determine whether to grant a first thread access to said object.

7. The method of claim 6, wherein the evaluating step utilizes an atomic compare and swap operation that determines if said portion of said lock is set to said predetermined value that indicates that the object is not locked by any thread, and if so, stores the thread identifier associated with said first thread as said thread identifier field of said lock to thereby grant said first thread access to said object.

8. The method of claim 7, wherein, if said compare and swap operation determines that said portion of said lock is not set to said predetermined value that indicates that the object is not locked by any thread, determining whether said thread identifier field of said lock is associated with another thread other than said first thread, and if so, obtaining a global lock.

9. The method of claim 8, wherein, upon obtaining said global lock, a second atomic operation is performed that sets said flag field of said lock to said predetermined value which indicates the there is a non-empty queue of waiting threads associated with the object.

10. The method of claim 9, wherein said second atomic operation comprises an atomic compare and swap operation.

11. The method of claim 9, wherein, upon successful completion of said second atomic operation, the thread identifier associated with said first thread is added to a queue of waiting threads associated with said object, said global lock is released and said first thread is suspended.

12. The method of claim 1, further comprising the step of evaluating said lock associated with said object in order to determine whether to unlock access to said object.

13. The method of claim 12, wherein, when a first thread issues an operation to unlock access to said object, the evaluating step utilizes an atomic compare-and-swap operation to determine if said thread identifier field of said lock associated with said object corresponds to said first thread and said flag field of said lock is set to a predetermined value that indicates that no other threads are waiting to lock the object.

14. The method of claim 12, wherein if said compare-and-swap operation determines that said thread identifier field of said lock corresponds to said first thread and said flag field of said lock is set to a predetermined value that indicates that no other threads are waiting to lock the object, said compare and swap operation updates said lock to indicate that said object is not locked by any thread.

15. The method of claim 12, further comprising the step of:

upon determining that said thread identifier field of said lock corresponds to said first thread and said flag field of said lock is not set to a predetermined value that indicates that no other threads are waiting to lock the object, obtaining a global lock.

16. The method of claim 15, further comprising the steps of:

upon obtaining said global lock, evaluating said queue associated with said object to determine if no other threads are waiting to access said object; and

upon determining that no other threads are waiting to access said object, updating said lock associated with said object to indicate that said object is not locked by any thread, and releasing said global lock.

17. The method of claim 15, further comprising the steps of:

upon obtaining said global lock, evaluating said queue associated with said object to determine if other threads are waiting to access said object; and

upon determining that other threads are waiting to access said object, updating said queue to remove a second thread from said queue, resuming said second thread and releasing said global lock.

18. The method of claim 17, further comprising the steps of:

upon updating said queue to remove said second thread, determining if said queue is empty;

if it is determined that said queue is empty, updating said lock such that said thread identifier field of said lock is set to a thread identifier corresponding to said second thread and said flag field is set to said predetermined value that indicates that no other threads are waiting to lock the object; and

if it is determined that said queue is non-empty, updating said lock such that said thread identifier field of said lock is set to a thread identifier corresponding to said second thread and said flag field is set to said predetermined value that indicates that there is a non-empty queue of waiting threads associated with the object.

19. A program storage device readable by a machine, tangibly embodying a program of instructions executable by the machine to perform method steps for controlling concurrent access to an object in a multi-threaded computer processing system, said method steps comprising:

storing in memory a lock that is associated with said object, said lock comprising a thread identifier field and a flag field;

wherein, when said object is not locked by any thread,

a portion of sail lock is set to a predetermined value that indicates that the object is not locked by any thread;

wherein, when an object is locked by a particular thread,

said thread identifier field is set to a value that identifies the particular thread,

if no other threads are waiting to lock the object, said flag field is set to a predetermined value that indicates that no other threads are waiting to lock the object, and

if other threads are waiting to lock the object, said flag field is set to a predetermined value which indicates that there is a non-empty queue of waiting threads associated with the object.

20. The program storage device of claim 19, wherein, when said object is not locked by any thread, said portion of said lock that is set to a predetermined value that indicates that the object is not locked by any thread comprises said thread identifier field and flag field of said lock.

21. The program storage device of claim 19, wherein, when said object is not locked by any thread, said portion of said lock that is set to a predetermined value that indicates that the object is not locked by any thread comprises said thread identifier field of said lock.

22. The program storage device of claim 19, wherein said flag field is a single bit.

23. The program storage device of claim 19, wherein said thread identifier field and said flag field comprise a word having a more significant portion distinct from a less significant portion, wherein said flag field is stored in said less significant portion and said thread identifier field is stored in said more significant portion.

24. The program storage device of claim 19, further comprising the step of evaluating said lock associated with said object in order to determine whether to grant a first thread access to said object.

25. The program storage device of claim 24, wherein the evaluating step utilizes an atomic compare and swap operation that determines if said portion of said lock is set to said predetermined value that indicates that the object is not locked by any thread, and if so, stores the thread identifier associated with said first thread as said thread identifier field of said lock to thereby grant said first thread access to said object.

26. The program storage device of claim 25, wherein, if said compare and swap operation determines that said thread identifier field is not set to said predetermined value that indicates that the object is not locked by any thread, determining whether said thread identifier field of said lock is associated with another thread other than said first thread, and if so, obtaining a global lock.

27. The program storage device of claim 26, wherein, upon obtaining said global lock, a second atomic operation is performed that sets said flag field of said lock to said predetermined value which indicates the there is a non-empty queue of waiting threads associated with the object.

28. The program storage device of claim 27, wherein said second atomic operation comprises an atomic compare and swap operation.

29. The program storage device of claim 27, wherein, upon successful completion of said second atomic operation, the thread identifier associated with said first thread is added to a queue of waiting threads associated with said object, said global lock is released and said first thread is suspended.

30. The program storage device of claim 19, further comprising the step of evaluating said lock associated with said object in order to determine whether to unlock access to said object.

31. The program storage device of claim 30, wherein, when a first thread issues an operation to unlock access to said object, the evaluating step utilizes an atomic compare-and-swap operation to determine if said thread identifier field of said lock associated with said object corresponds to said first thread and said flag field of said lock is set to a predetermined value that indicates that no other threads are waiting to lock the object.

32. The program storage device of claim 31, wherein if said compare-and-swap operation determines that said thread identifier field of said lock corresponds to said first thread and said flag field of said lock is set to a predetermined value that indicates that no other threads are waiting to lock the object, said compare and swap operation updates said lock to indicate that said object is not locked by any thread.

33. The program storage device of claim 30, further comprising the step of:

upon determining that said thread identifier field of said lock corresponds to said first thread and said flag field of said lock is not set to a predetermined value that indicates that no other threads are waiting to lock the object, obtaining a global lock.

34. The program storage device of claim 33, further comprising the steps of:

upon obtaining said global lock, evaluating said queue associated with said object to determine if no other threads are waiting to access said object; and

upon determining that no other threads are waiting to access said object, updating said lock associated with said object to indicate that said object is not locked by any thread, and releasing said global lock.

35. The program storage device of claim 33, further comprising the steps of:

upon obtaining said global lock, evaluating said queue associated with said object to determine if other threads are waiting to access said object; and

upon determining that other threads are waiting to access said object, updating said queue to remove a second thread from said queue, resuming said second thread and releasing said global lock.

36. The program storage device of claim 35, further comprising the steps of:

upon updating said queue to remove said second thread, determining if said queue is empty;

if it is determined that said queue is empty, updating said lock such that said thread identifier field of said lock is set to a thread identifier corresponding to said second thread and said flag field is set to said predetermined value that indicates that no other threads are waiting to lock the object; and

if it is determined that said queue is non-empty, updating said lock such that said thread identifier field of said lock is set to a thread identifier corresponding to said second thread and said flag field is set to said predetermined value that indicates that there is a non-empty queue of waiting threads associated with the object.

37. A method for controlling concurrent access to an object in a multi-threaded computer processing system, the method comprising the steps of:

storing in memory a lock that is associated with said object, said lock comprising a first flag field and a second flag field;

wherein, when said object is not locked by any thread,

a portion of said lock is set to a predetermined value that indicates that the object is not locked by any thread;

wherein, when an object is locked by a particular thread,

said first flag field is set to a value that indicates that the object is locked by a thread,

if no other threads are waiting to lock the object, said second flag field is set to a predetermined value that indicates that no other threads are waiting to lock the object, and

if other threads are waiting to lock the object, said second flag field is set to a predetermined value which indicates that there is a non-empty queue of waiting threads associated with the object.

38. The method of claim 37, wherein, when said object is not locked by any thread, said portion of said lock that is set to a predetermined value that indicates that the object is not locked by any thread comprises said first flag field and said second flag field of said lock.

39. The method of claim 37, wherein, when said object is not locked by any thread, said portion of said lock that is set to a predetermined value that indicates that the object is not locked by any thread comprises said first flag field of said lock.

40. The method of claim 37, wherein said first and second flag fields each comprise a single bit.

41. The method of claim 37, further comprising the step of evaluating said lock associated with said object in order to determine whether to grant a first thread access to said object, wherein the evaluating step utilizes an atomic compare and swap operation that determines if said portion of said lock is set to said predetermined value that indicates that the object is not locked by any thread, and if so, updates said first flag field of said lock to indicate that the object is locked by a thread to thereby grant said first thread access to said object.

42. A program storage device readable by a machine, tangibly embodying a program of instructions executable by the machine to perform method steps for controlling concurrent access to an object in a multi-threaded computer processing system, said method steps comprising:

storing in memory a lock that is associated with said object, said lock comprising a first flag field and a second flag field;

wherein, when said object is not locked by any thread,

a portion of said lock is set to a predetermined value that indicates that the object is not locked by any thread;

wherein, when an object is locked by a particular thread,

said first flag field is set to a value that indicates that the object is locked by a thread,

if no other threads are waiting to lock the object, said second flag field is set to a predetermined value that indicates that no other threads are waiting to lock the object, and

if other threads are waiting to lock the object, said second flag field is set to a predetermined value which indicates that there is a non-empty queue of waiting threads associated with the object.

43. The program storage device of claim 42, wherein, when said object is not locked by any thread, said portion of said lock that is set to a predetermined value that indicates that the object is not locked by any thread comprises said first flag field and said second flag field of said lock.

44. The program storage device of claim 42, wherein, when said object is not locked by any thread, said portion of said lock that is set to a predetermined value that indicates that the object is not locked by any thread comprises said first flag field of said lock.

45. The program storage device of claim 42, wherein said first and second flag fields each comprise a single bit.

46. The program storage device of claim 42, further comprising the step of evaluating said lock associated with said object in order to determine whether to grant a first thread access to said object, wherein the evaluating step utilizes an atomic compare and swap operation that determines if said portion of said lock is set to said predetermined value that indicates that the object is not locked by any thread, and if so, updates said first flag field of said lock to indicate that the object is locked by a thread to thereby grant said first thread access to said object.