Method and apparatus for serializing access to a shared resource in an information handling system

Abstract

A method and apparatus for serializing access by n entities to a shared resource in an information handling system. A waiter list is defined as a circular list of n bits, each of which is assigned to an entity. When a bit is false (0) it indicates that the corresponding entity is not waiting for the lock; when the bit is true (1) it indicates the corresponding entity is waiting for the lock. A next waiter indicator (NWI) is also defined that contains a value from 0 to n inclusive; a value of 0 indicates that there are currently no waiters, while a value from 1 to n indicates the next waiter to whom the lock will be granted. The waiter list is initialized to zeros to indicate there are no waiters. When an entity requests a lock that cannot be granted, the entity is made a waiter by setting the corresponding bit in the waiter list to one. If the next waiter indicator is zero, indicating that there were previously no waiters for the lock, the indicator is set to identify the requesting entity as the next waiter. When a lock is released and there is a next waiter for the lock as indicated by the next waiter indicator, that waiter is made a lock holder and the corresponding bit in the waiter list is reset to indicate that the entity is no longer a waiter for the lock. A new next waiter is then found by scanning the waiter list beginning with the entry following that for the entity granted access to the resource. This ensures that once a waiter releases a lock, the waiter is not selected to receive the lock again until all other waiters have received the lock.