STREAMS drivers configures the degree of concurrency using the cb_flag field in the cb_ops structure (see cb_ops(9S)). The corresponding field for STREAMS modules is the f_flag in the fmodsw structure.
For the purpose of restricting and controlling the concurrency in drivers/modules, we define the concepts of inner and outer perimeters. A driver/module can
be configured either to have no perimeters, to have only an inner or an outer perimeter, or to have both an inner and an outer perimeter. Each perimeter acts as a readers-writers lock, that is, there
can be multiple concurrent readers or a single writer. Thus, each perimeter can be entered in two modes: shared (reader) or exclusive (writer). The mode depends on the perimeter configuration and can be
different for the different STREAMS entry points ( open(9E), close(9E), put(9E), or srv(9E)).
The concurrency for the different entry points is (unless specified otherwise) to enter with exclusive access at the inner perimeter (if present) and shared access at the outer perimeter (if present).
The perimeter configuration consists of flags that define the presence and scope of the inner perimeter, the presence of the outer perimeter (which can only have one scope), and flags that modify
the default concurrency for the different entry points.
All MT safe modules/drivers specify the D_MP flag.
Inner Perimeter Flags
The inner perimeter presence and scope are controlled by the mutually exclusive flags:
- The module/driver has an inner perimeter around each queue.
- The module/driver has an inner perimeter around each read/write pair of queues.
- The module/driver has an inner perimeter that encloses all the module's/driver's queues.
- None of the above
- The module/driver has no inner perimeter.
Outer Perimeter Flags
The outer perimeter presence is configured using:
- In addition to any inner perimeter, the module/driver has an
outer perimeter that encloses all the module's/driver's queues. This can be combined with all the inner perimeter options except D_MTPERMOD.
The default concurrency can be modified using:
- This flag modifies the default behavior when put(9E) procedure are invoked so that the inner perimeter is entered shared instead of exclusively.
- This flag modifies the default behavior when open(9E) and close(9E) procedures are invoked so the the outer perimeter is entered exclusively instead of shared.
The module/driver can use qwait(9F) or qwait_sig() in the open(9E) and close(9E) procedures if it needs to wait "outside" the perimeters.
The module/driver can use qwriter(9F) to upgrade the access at the inner or outer
perimeter from shared to exclusive.
The use and semantics of qprocson() and qprocsoff(9F) is independent
of the inner and outer perimeters.