Mutual Exclusion Primitives

Mutual Exclusion Primitives

IRIX supports library functions that perform atomic (uninterruptable) sample-and-set operations on words of memory. For example, test_and_set() copies the value of a word and stores a new value into the word in a single operation; while test_then_add() samples a word and then replaces it with the sum of the sampled value and a new value.

These primitive operations can be used as the basis of mutual-exclusion protocols using words of shared memory. For details, see the test_and_set(3p) reference page.

The test_and_set() and related functions are based on the MIPS R4000 instructions Load Linked and Store Conditional. Load Linked retrieves a word from memory and tags the processor data cache "line" from which it comes. The following Store Conditional tests the cache line. If any other processor or device has modified that cache line since the Load Linked was executed, the store is not done. The implementation of test_then_add() is comparable to the following assembly-language loop:

1:
    ll    retreg, offset(targreg)
    add   tmpreg, retreg, valreg
    sc    tmpreg, offset(targreg)
    beq   tmpreg, 0, b1

The loop continues trying to load, augment, and store the target word until it succeeds. Then it returns the value retrieved. For more details on the R4000 machine language, see one of the books listed in "Other Useful Books" on page xxiii.

The Load Linked and Store Conditional instructions only operate on memory locations that can be cached. Uncached pages (for example, pages implemented as reflective shared memory, see "Reflective Shared Memory") cannot be set by the test_and_set() functions.