Read lock and write lock management system based upon mutex and semaphore availability

• US 6,029,190 A
• Filed: 09/24/1997
• Issued: 02/22/2000
• Est. Priority Date: 09/24/1997
• Status: Expired due to Term

• Current Assignee: Sony Corporation (Sony Group Corp.), Sony Pictures Entertainment Incorporated (Sony Group Corp.)
• Original Assignee: Sony Corporation (Sony Group Corp.), Sony Electronics Inc. (Sony Group Corp.)
• Inventors: Oliver, Richard J.
• Primary Examiner(s): Banankhah, Majid A.
• Application Number:
  US08/937,094
• Time in Patent Office: 881 Days
• Field of Search: 709/101, 709/104, 709/107, 709/400, 709/102
• US Class Current: 718/107
• CPC Class Codes: G06F 9/526 Mutual exclusion algorithms

Abstract

A method and apparatus for implementing a read/write lock mechanism in software. In an operating environment that allows objects such threads to wait on a plurality of synchronization objects simultaneously, a read/write lock can be implemented using two synchronization objects such as a mutex and a semaphore. Further, a variable is introduced to track the number of reader threads awaiting to read protected data. The read/write lock permits a plurality of reader threads to access protected data simultaneously, while only allowing a single writer thread access to a protected data location.

Claims

1. A method for simulating a read/write lock comprising:

obtaining a read lock for a plurality of reader threads given a first condition occurs, the first condition comprising the availability of a mutex, said mutex comprising a single object that can be waited upon simultaneously by said plurality of reader threads and a plurality of writer threads; and

obtaining a write lock for a single writer thread given that a second condition occurs, the second condition comprising the simultaneous availability of a mutex and a semaphore, said semaphore comprising a single object that can be waited upon simultaneously by a plurality of writer threads.

2. A method according to claim 1, wherein obtaining a read lock comprises:

obtaining said mutex when available;

incrementing a first variable to track the number of reader threads active;

testing a third condition, said third condition comprising said first variable being equal to one;

releasing said mutex; and

reading data from a location protected by said read lock.

3. A method according to claim 2 wherein if said third condition is true, said semaphore is obtained, and if said third condition is false, said semaphore is not obtained.

4. A method according to claim 1, further comprising:

releasing the obtained read lock by one of said plurality of reader threads; and

releasing the obtained write lock by said single writer thread.

5. A method according to claims 1, wherein obtaining a write lock comprises:

obtaining said mutex and said semaphore when said mutex and said semaphore are both available;

releasing said semaphore; and

writing to a data location protected by said write lock.

6. A method according to claim 4, wherein releasing the read lock comprises:

obtaining said mutex when available;

decrementing said first variable;

testing a fourth condition, said fourth condition comprising said first variable being equal to zero; and

releasing said mutex.

7. A method according to claim 6 wherein if said fourth condition is true, said semaphore is released.

8. A method according to claim 4, wherein releasing a write lock comprises releasing said mutex.

9. A computer system comprising:

a processor;

a memory coupled to said processor, said processor configured to;

obtain a read lock for a plurality of reader threads given a first condition occurs, the first condition comprising the availability of a mutex, said mutex comprising a single object that can be waited upon simultaneously by said plurality of reader threads and a plurality of writer threads; and

obtain a write lock for a single writer thread given that a second condition occurs, the second condition comprising the simultaneous availability of a mutex and a semaphore, said semaphore comprising a single object that can be waited upon simultaneously by a plurality of writer threads.

18. The computer system according to claim 9, wherein the processor is configured to obtain a read lock by obtaining said mutex when available;

incrementing a first variable to track the number of reader threads active;

testing a third condition, said third condition comprising said first variable being equal to one;

releasing said mutex; and

reading data from a location protected by said read lock.

19. The computer system according to claim 9, wherein the processor is further configured to release the obtained read lock by one of said plurality of reader threads;

and release the obtained write lock by said single writer thread.

20. The computer system according to claims 9, wherein the processor is configured to obtain a write lock by obtaining said mutex and said semaphore when said mutex and said semaphore are both available;

releasing said semaphore; and

writing to a data location protected by said write lock.

10. A computer readable medium comprising instructions, which when executed by a processor, perform the steps for simulating read/write lock comprising:

obtaining a read lock for a plurality of reader threads given a first condition occurs, the first condition comprising the availability of a mutex, said mutex comprising a single object that can be waited upon simultaneously by said plurality of reader threads and a plurality of writer threads; and

obtaining a write lock for a single writer thread given that a second condition occurs, the second condition comprising the simultaneous availability of a mutex and a semaphore, said semaphore comprising a single object that can be waited upon simultaneously by a plurality of writer threads.

11. The computer readable medium according to claim 10, further comprising instructions, which when executed obtain a read lock by:

obtaining said mutex when available;

incrementing a first variable to track the number of reader threads active;

testing a third condition, said third condition comprising said first variable being equal to one;

releasing said mutex; and

reading data from a location protected by said read lock.

12. The computer readable medium according to claim 11, wherein if said third condition is true, said semaphore is obtained, and if said third condition is false, said semaphore is not obtained.

13. The computer readable medium according to claim 10, further comprising instructions which when executed, release the obtained read lock by one of said plurality of reader threads, and release the obtained write lock by said single writer thread.

14. A computer readable medium according to claims 10, wherein obtaining a write lock comprises obtaining said mutex and said semaphore when said mutex and said semaphore are both available;

releasing said semaphore; and

writing to a data location protected by said write lock.

15. A computer readable medium according to claim 13, wherein releasing the read lock comprises obtaining said mutex when available;

decrementing said first variable;

testing a fourth condition, said fourth condition comprising said first variable being equal to zero; and

releasing said mutex.

16. A computer readable medium according to claim 12, wherein releasing a write lock comprises releasing said mutex.

17. The computer readable medium according to claim 15, wherein if said fourth condition is true, said semaphore is released.

21. A computer system comprising:

a processor configured to execute a plurality of threads;

a memory coupled to the processor;

a mutex comprising a single object that can be waited upon concurrently by a plurality of reader thread and a plurality of writer threads;

a semaphore comprising a single object that can be waited upon concurrently by a plurality of writer threads;

a read lock for a plurality of reader threads, said read lock obtained by a thread if a given first condition comprising the availability of a mutex occurs; and

a write lock for a single writer thread if a second condition comprising the concurrent availability of the mutex and the semaphore occurs.

22. The computer system according to claim 21, further comprising a first variable to track the number of reader threads active.