In
recent years, that is in past 5 years Linux has seen significant growth as a server
operating system and has been successfully deployed as an enterprise for Web,
file and print servicing. With the advent of Kernel Version 2.4, Linux has seen
a tremendous boost in scalability and robustness which further makes it feasible
to deploy even more demanding enterprise applications such as high end database,
business intelligence software ,application servers, etc. As a result, whole enterprise
business suites and middleware such as SAP, Websphere, Oracle, etc., are now available
on Linux. For these enterprise applications to
run efficiently on Linux, or on any other operating system, the OS must provide
the proper abstractions and services. Usually these enterprise applications and
applications suites or software are increasingly built as multi process / multithreaded
applications. These application suites are often a collection of multiple independent
subsystems. Despite functional variations between these applications often they
require to communicate with each other and also sometimes they need to share a
common state.
Examples of this are database systems, which typically maintain
shared I/O buffers in user space.Access to such
shared state must be properly synchronized. Allowing multiple processes to access
the same resources in a time sliced manner or potentially consecutively in the
case of multiprocessor systems can cause many problems. This is due to the need
to maintain data consistency, maintain true temporal dependencies and to ensure
that each thread will properly release the resource as required when it has completed
its action. Synchronization can be established through locks. There are mainly
two types of locks: - Exclusive locks and shared locks. Exclusive locks are those
which allows only a single user to access the protected entity, while shared locks
are those which implements the multiple reader - single writer semantics.
