Lock Granularity

Here are some issues to consider when deciding on how many locks to use in a driver:

• The driver should allow as many threads as possible into the driver: this leads to fine-grained locking.

• However, it should not spend too much time executing the locking primitives: this approach leads to coarse-grained locking.

• Driver code should be simple and maintainable.

• Avoid lock contention for shared data.

• Write re-entrant code wherever possible. This makes it possible for many threads to execute without grabbing any locks.

• Use locks to protect the data and not the code path.

• Keep in mind the level of concurrency provided by the device; if the controller can only handle one request at a time, there is no point in spending excessive time making the driver handle multiple threads.

A little thought in reorganizing the ordering and types of locks around such data can lead to considerable savings.