The desired degree of concurrency
The natural concurrency of the underlying module
The amount of effort or complexity required
Much of the effort in conversion is simply determining the appropriate degree of data sharing and the corresponding granularity of locking (see Table 12-1). The actual time spent configuring perimeters and/or installing locks should be much smaller than the time spent in analysis.
To port your module, you must understand the data structures used within your module, as well as the accesses to those data structures. You must fully understand the relationship between all portions of the module and private data within that module, and to use the MT STREAMS perimeters (or the synchronization primitives available) to maintain the integrity of these private data structures.
You must explicitly restrict access to private module data structures as appropriate to ensure the integrity of these data structures. You must use the MT STREAMS perimeters to restrict the concurrency in the module so that the parts of the module that modify private data are single-threaded with respect to the parts of the module that read the same data. (For more information about perimeters, see MT STREAMS Perimeters.) Besides perimeters, you can use the synchronization primitives available (mutex, condition variables, readers/writer, semaphore) to explicitly restrict access to module private data appropriate for the operations within the module on that data.
The first step in multithreading a module or driver is to analyze the module, breaking the entire module up into a list of individual operations and the private data structures referenced in each operation. Part of this first step is deciding upon a level of concurrency for the module. Ask yourself which of these operations can be multithreaded and which must be single-threaded. Try to find a level of concurrency that is “natural” for the module and matches one of the available perimeters (or, alternatively, requires the minimal number of locks) , and has a simple and straightforward implementation. Avoid additional unnecessary complexity.
Typical questions to ask are:
What data structures are maintained within the module?
What types of accesses are made to each field of these data structures?
When is each data structure accessed destructively (written) and when is it accessed non-destructively (read)?
Which operations within the module should be allowed to execute concurrently?
Is per module single-threading appropriate for the module?
Is per queue-pair or per queue single-threading appropriate?
What are the message ordering requirements?
The SunOS 5 Device Driver Interface (DDI/DKI)
The SunOS 5 MT design
For portability and correct operation, each module must adhere to the SunOS DDI/DKI. Several facilities available in previous releases of the SunOS system have changed and can take different arguments, or produce different side effects, or no longer exist in the SunOS 5 system. The module writer should carefully review the module with respect to the DDI/DKI.
Each module that accesses underlying Sun-specific features included in the SunOS 5 system should conform to the Device Driver Interface. The SunOS 5 DDI defines the interface used by the device driver to register device hardware interrupts, access device node properties, map device slave memory, and establish and synchronize memory mappings for DVMA (Direct Virtual Memory Access). These areas are primarily applicable to hardware device drivers. Refer to the Device Driver Interface Specification within the Writing Device Drivers for details on the SunOS 5 DDI and DVMA.
The kernel networking subsystem in the SunOS 5 system is based on STREAMS. Datalink drivers that used the ifnet interface in the SunOS 4 system must be converted to use DLPI for the SunOS 5 system. Refer to the Data Link Provider Interface, Revision 2 specification.
After reviewing the module for conformance to the SunOS 5 DKI and DDI specifications, you should be able to consider the impact of multithreading on the module.