STREAMS/UX for the HP 9000 Reference Manual

STREAMS/UX Multiprocessor Support

Writing MP Scalable Modules and Drivers

spinlocks when calling some STREAMS/UX utilities. See Table 1 at the end of

this chapter for more information. See the SVR4.2 Driver manual for more

information about SVR4 MP hierarchies.

• To reduce contention and improve performance, you should minimize the

amount of time that modules and drivers hold spinlocks.

• To improve performance, modules and drivers should verify that they are actually

running on a multiprocessor system before calling the HP-UX native spinlock

primitives. The SVR4 MP LOCK and UNLOCK routines described in Chapter

3 do this for the caller. If a spinlock is being used only to protect against software

running on other processors, but not interrupts, modules or drivers can call the

MP_SPINLOCK and MP_SPINUNLOCK macros in /usr/include/sys/spinlock.h

(or /usr/conf/h/spinlock.h). These macros obtain only the requested spinlock if

they are executing on a multiprocessor system. If a spinlock is being used to

protect against both software running on other processors and interrupts, modules

and drivers should check the uniprocessor flag and raise the spl level if they are

running on a uniprocessor system. Example code is shown below.

if (uniprocessor)

x = splstr();

else

spinlock(mylock);

• Be careful when choosing a multiplexor's synchronization level. When a driver

is linked under a mux, STREAMS/UX changes the driver's Stream head to be the

lower mux. STREAMS/UX uses the upper mux's synchronization level for the

lower mux. So if the upper mux uses global, elsewhere, or module

synchronization, the lower and upper muxes can share data. If the upper mux

uses queue or queue pair synchronization, the lower and upper muxes cannot

share data.

The synchronization level also inﬂuences how messages can be passed across

the mux. If the upper mux uses global, elsewhere, or module synchronization, it

can pass messages downward by passing the lower mux's write queue to putq,

put, or putnext. Likewise, the lower mux can pass messages upward by passing

the upper mux's read queue to putq, put, or putnext. If the upper mux uses queue

or queue pair synchronization, it can only use putq and put to pass messages to

the lower mux. To use putnext, the upper mux must ensure that the driver stays

linked under the mux until after the putnext completes. Also, the lower mux can

only use putq and put to pass messages to the upper mux. To use putnext, the

lower mux must guarantee that the driver stays linked under the mux, that the

mux stays open, and that modules are not pushed or popped until after the

putnext completes.