READ/WRITE LOCK WITH REDUCED READER LOCK SAMPLING OVERHEAD IN ABSENCE OF WRITER LOCK ACQUISITION

• US 20080320262A1
• Filed: 06/22/2007
• Published: 12/25/2008
• Est. Priority Date: 06/22/2007
• Status: Active Grant

• Current Assignee: International Business Machines Corporation
• Original Assignee: International Business Machines Corporation
• Inventors: Srivatsa, Vaddagiri, McKenney, Paul E., Shenoy, Gautham R.
• Granted Patent: US 7,934,062 B2
• Time in Patent Office: Days
• US Class Current: 711/163
• CPC Class Codes: G06F 2209/523 Mode
• CPC Class Codes: G06F 9/52 Program synchronisation; Mutual exclusion, e.g. by means of semaphores
• CPC Class Codes: G06F 9/526 Mutual exclusion algorithms

Abstract

An improved reader-writer locking for synchronizing access to shared data. When writing the shared data, a writer flag is set and a lock is acquired on the shared data. The shared data may be accessed following the expiration of a grace period and a determination that there are no data readers accessing the shared data. When reading the shared data, the writer flag is tested that indicates whether a data writer is attempting to access the shared data. If the writer flag is not set, the shared data is accessed using a relatively fast read mechanism. If the writer flag is set, the shared data is accessed using a relatively slow read mechanism.

Claims

1. A method for synchronizing access to shared data, comprising:

when reading said shared data;

testing a writer flag that indicates whether a data writer is attempting to access said shared data;

accessing said shared data using a relatively fast read mechanism if said writer flag is not set; and

accessing said shared data using a relatively slow read mechanism if said writer flag is set; and

when writing said shared data;

setting said writer flag and acquiring a lock on said shared data;

waiting for expiration of a grace period;

waiting until there are no data readers accessing said shared data using said relatively fast read mechanism; and

accessing said shared data.

2. A method in accordance with claim 1 wherein said relatively fast read mechanism comprises incrementing a reader reference counter prior to a data reader accessing said shared data and decrementing said reader reference counter after said data reader accesses said shared data.

3. A method in accordance with claim 1 wherein said relatively slow read mechanism comprises acquiring said lock prior to a data reader accessing said shared data and releasing said lock after said data reader accesses said shared data.

4. A method in accordance with claim 1 wherein said grace period guarantees that a said writer flag will be seen in a set condition if said writer flag is tested after said writer flag is set.

5. A method in accordance with claim 2 wherein said grace period guarantees that said incrementing or decrementing of said reader reference counter will complete prior to determining that no data readers are accessing said shared data if said writer flag is tested before said writer flag is set.

6. A method in accordance with claim 1 wherein said reader reference counters are tested to determine that there are no data readers accessing said shared data.

7. A method in accordance with claim 1 wherein acquiring said lock is effectuated by setting said writer flag.

8. A data processing system having one or more processors, a memory and a communication pathway between the one or more processors and the memory, said system being adapted to synchronize access to shared data, and comprising:

a writer flag that indicates whether a data writer is attempting to access said shared data;

fast path read logic adapted to coordinate relatively fast reader access to said shared data on behalf of a data reader when said writer flag is not set;

slow path read logic adapted to coordinate relatively slow reader access to said shared data on behalf of said data reader when said writer flag is set;

first write logic adapted to set said writer flag and acquire a lock on said shared data on behalf of said data writer;

grace period processing logic adapted to implement a grace period following said writer flag being set; and

second write logic adapted to wait until there are no data readers accessing said shared data using said fast read logic, and thereafter allow said data writer to access said shared data.

9. A system in accordance with claim 8 wherein said fast read logic is adapted to increment a reader reference counter prior to said data reader accessing said shared data and to decrement said reader reference counter after said data reader accesses said shared data.

10. A system in accordance with claim 8 wherein said slow read logic is adapted to acquire said lock prior to accessing said shared data and releasing said lock after accessing said shared data.

11. A system in accordance with claim 8 wherein said grace period guarantees that said data reader will see said writer flag in a set condition if said data reader tests said writer flag after said writer flag is set.

12. A system in accordance with claim 9 wherein said grace period guarantees that said incrementation or decrementation of said reader reference counter will complete prior to said second write logic determining that no data readers are accessing said shared data if said writer flag is tested before said writer flag is set.

13. A system in accordance with claim 8 wherein said second writer logic tests said reader reference counters to determine that there are no data readers accessing said shared data.

14. A computer program product for synchronizing access to shared data, comprising:

one or more machine-readable media;

means provided by said one or more media for programming a data processing platform to operate as by;

when reading said shared data;

testing a writer flag that indicates whether a data writer is attempting to access said shared data;

accessing said shared data using a relatively fast read mechanism if said writer flag is not set; and

accessing said shared data using a relatively slow read mechanism if said writer flag is set; and

when writing said shared data;

setting said writer flag and acquiring a lock on said shared data;

waiting for expiration of a grace period;

waiting until there are no data readers accessing said shared data using said relatively fast read mechanism; and

accessing said shared data.

15. A computer program product in accordance with claim 14 wherein said relatively fast read mechanism comprises incrementing a reader reference counter prior to a data reader accessing said shared data and decrementing said reader reference counter after said data reader accesses said shared data.

16. A computer program product in accordance with claim 14 wherein said relatively slow read mechanism comprises acquiring said lock prior to a data reader accessing said shared data and releasing said lock after said data reader accesses said shared data.

17. A computer program product in accordance with claim 14 wherein said grace period guarantees that said writer flag will be seen in a set condition if said writer flag is tested after said writer flag is set.

18. A computer program product in accordance with claim 15 wherein said grace period guarantees that said incrementing or decrementing of said reader reference counter will complete prior to determining that no data readers are accessing said shared data if said writer flag is tested before said writer flag is set.

19. A computer program product in accordance with claim 14 wherein said reader reference counters are tested to determine that there are no data readers accessing said shared data.

20. A computer program product for synchronizing access to shared data, comprising:

one or more machine-readable media;

means provided by said one or more media for programming a data processing platform to operate as by;

setting a writer flag and acquiring a lock on said shared data;

waiting for expiration of a grace period;

waiting until there are no data readers accessing said shared data; and

accessing said shared data.