Planning for Concurrency

Planning for Concurrency

Since the ULI handler can interrupt the program at any point, or run concurrently with it, the program must be prepared for concurrent execution. There are two areas to consider: global variables, and library routines.

Debugging With Interrupts

The asynchronous, possibly concurrent entry to the ULI handler can confuse a debugging monitor such as dbx. Some strategies for dealing with this are covered in the uli(3) reference page.

Declaring Global Variables

When variables can be modified by both the main process and the ULI handler, you must take special care to avoid race conditions.

An important step is to specify -D_SGI_REENTRANT_FUNCTIONS to the compiler, so as to get the reentrant versions of the C library functions. This ensures that, if the main process and the ULI handler both enter the C library, there will be no collision over global variables.

You can declare the global variables that are shared with the ULI handler with the keyword "volatile," so that the compiler will generate code to load the variables from memory on each reference. However, the compiler never holds global values in registers over a function call, and you almost always have a function call (such as ULI_block_intr()) preceding a test of a shared global variable.