RAS(9)							   BSD Kernel Developer's Manual						    RAS(9)

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 check-
     ing 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 compo-
     nent in cpu_switchto(9).  A port which supports restartable atomic sequences will define __HAVE_RAS in <machine/types.h> for machine-indepen-
     dent 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 reg-
	      istered 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.

BSD
								  April 17, 2010							       BSD