mt-streams(9F) Kernel Functions for Drivers mt-streams(9F)
mt-streams - STREAMS multithreading
Solaris DDI specific (Solaris DDI).
STREAMS drivers configure 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 driv‐
ers/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 exclu‐
sive (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 con‐
currency 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
D_MTPERQ The module/driver has an inner perimeter around
D_MTQPAIR The module/driver has an inner perimeter around
each read/write pair of queues.
D_MTPERMOD 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:
D_MTOUTPERIM In addition to any inner perimeter, the module/driver
has an outer perimeter that encloses all the mod‐
ule's/driver's queues. This can be combined with all
the inner perimeter options except D_MTPERMOD.
Note that acquiring exclusive access at the outer perimeter (that is,
using qwriter(9F) with the PERIM_OUTER flag) can incur significant per‐
formance penalties, which grow linearly with the number of open
instances of the module or driver in the system.
The default concurrency can be modified using:
D_MTPUTSHARED This flag modifies the default behavior when put(9E)
procedure are invoked so that the inner perimeter is
entered shared instead of exclusively.
D_MTOCEXCL This flag modifies the default behavior when open(9E)
and close(9E) procedures are invoked so the outer
perimeter is entered exclusively instead of shared.
Note that drivers and modules using this flag can
cause significant system performance degradation dur‐
ing stream open or close when many instances of the
driver or module are in use simultaneously. For this
reason, use of this flag is discouraged. Instead,
since open(9E) and close(9E) both execute with user
context, developers are encouraged to use traditional
synchronization routines such as cv_wait_sig(9F) to
coordinate with other open instances of the module or
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.
close(9E), open(9E), put(9E), srv(9E), qprocsoff(9F), qprocson(9F),
qwait(9F), qwriter(9F), cb_ops(9S)
STREAMS Programming Guide
Writing Device Drivers
SunOS 5.11 5 May 2009 mt-streams(9F)