NAME

ras_lookup, ras_fork, ras_purgeall — restartable atomic sequences

SYNOPSIS

#include <sys/types.h>
#include <sys/proc.h>
#include <sys/ras.h>

void *
ras_lookup(struct proc *p, void *addr);

int
ras_fork(struct proc *p1, struct proc *p2);

int
ras_purgeall(struct proc *p);

DESCRIPTION

Restartable atomic sequences are user code sequences which are guaranteed to execute without preemption. This property is assured by checking the set of restartable atomic sequences registered for a process during cpu_switchto(9). If a process is found to have been preempted during a restartable sequence, then its execution is rolled-back to the start of the sequence by resetting its program counter saved in its process control block (PCB).

The RAS functionality is provided by a combination of the machine-independent routines discussed in this page and a machine-dependent component in cpu_switchto(9). A port which supports restartable atomic sequences will define __HAVE_RAS in <machine/types.h> for machine-independent code to conditionally provide RAS support.

A complicated side-effect of restartable atomic sequences is their interaction with the machine-dependent ptrace(2) support. Specifically, single-step traps and/or the emulation of single-stepping must carefully consider the effect on restartable atomic sequences. A general solution is to ignore these traps or disable them within restartable atomic sequences.

FUNCTIONS

The functions which operate on restartable atomic sequences are:

ras_lookup(p, addr)

This function searches the registered restartable atomic sequences for process p which contain the user address addr. If the address addr is found within a RAS, then the restart address of the RAS is returned, otherwise -1 is returned.

ras_fork(p1, p2)

This function is used to copy all registered restartable atomic sequences for process p1 to process p2. It is primarily called from fork1(9) when the sequences are inherited from the parent by the child.

ras_purgeall(p)

This function is used to remove all registered restartable atomic sequences for process p. It is primarily used to remove all registered restartable atomic sequences for a process during exec(3) and by rasctl(2).

CODE REFERENCES

The RAS framework itself is implemented within the file sys/kern/kern_ras.c. Data structures and function prototypes for the framework are located in <sys/ras.h>. Machine-dependent portions are implemented within cpu_switchto(9) in the machine-dependent file sys/arch/<arch>/<arch>/locore.S.

SEE ALSO

rasctl(2), cpu_switchto(9), fork1(9)

Gregory McGarry, An Implementation of User-level Restartable Atomic Sequences on the NetBSD Operating System, Proceedings of the FREENIX Track: 2003 USENIX Annual Technical Conference, USENIX Association, http://www.usenix.org/publications/library/proceedings/usenix03/tech/freenix03/full_papers/mcgarry/mcgarry.pdf, 311-322, June 9-14, 2003.

HISTORY

The RAS functionality first appeared in NetBSD 2.0.