NAME

rasctl — restartable atomic sequences

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

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

int
rasctl(void *addr, size_t len, int op);

DESCRIPTION

Restartable atomic sequences are code sequences which are guaranteed to execute without preemption. This property is assured by the kernel by re-executing a preempted sequence from the start. This functionality enables applications to build atomic sequences which, when executed to completion, will have executed atomically. Restartable atomic sequences are intended to be used on systems that do not have hardware support for low-overhead atomic primitives.

The rasctl function manipulates a process's set of restartable atomic sequences. If a restartable atomic sequence is registered and the process is preempted within the range addr and addr+len, then the process is resumed at addr.

As the process execution can be rolled-back, the code in the sequence should have no side effects other than a final store at addr+len-1. The kernel does not guarantee that the sequences are successfully restartable. It assumes that the application knows what it is doing. Restartable atomic sequences should adhere to the following guidelines:

have a single entry point and a single exit point;
not execute emulated instructions; and
not invoke any functions or system calls.

Restartable atomic sequences are inherited from the parent by the child during the fork(2) operation. Restartable atomic sequences for a process are removed during exec(3).

The operations that can be applied to a restartable atomic sequence are specified by the op argument. Possible operations are:

RAS_INSTALL: Install this sequence.
RAS_PURGE: Remove the specified registered sequence for this process.
RAS_PURGE_ALL: Remove all registered sequences for this process.

The RAS_PURGE and RAS_PURGE_ALL operations should be considered to have undefined behaviour if there are any other runnable threads in the address space which might be executing within the restartable atomic sequence(s) at the time of the purge. The caller must be responsible for ensuring that there is some form of coordination with other threads to prevent unexpected behaviour.

To preserve the atomicity of sequences, the kernel attempts to protect the sequences from alteration by the ptrace(2) facility.

RETURN VALUES

Upon successful completion, rasctl() returns zero. Otherwise, -1 is returned and errno is set to indicate the error.

ERRORS

The rasctl function will fail if:

[EINVAL]: Invalid input was supplied, such as an invalid operation, an invalid address, or an invalid length. A process may have a finite number of atomic sequences that is defined at compile time.
[EOPNOTSUPP]: Restartable atomic sequences are not supported by the kernel.
[ESRCH]: Restartable atomic sequence not registered.

HISTORY

The rasctl functionality first appeared in NetBSD 2.0 based on a similar interface that appeared in Mach 2.5.

CAVEATS

Modern compilers reorder instruction sequences to optimize speed. The start address and size of a RAS need to be protected against this. One level of protection is created by compiler dependent instructions, abstracted from user level code via the following macros:

RAS_DECL(name): Declares the start and end labels used internally by the other macros to mark a RAS. The name uniquely identifies the RAS.
RAS_START(name): Marks the start of the code. Each restart returns to the instruction following this macro.
RAS_END(name): Marks the end of the restartable code.
RAS_ADDR(name): Returns the start address of a RAS and is used to create the first argument to rasctl.
RAS_SIZE(name): Returns the size of a RAS and is used as second argument to rasctl.

Recent versions of gcc(1) require the -fno-reorder-blocks flag to prevent blocks of code wrapped with RAS_START/RAS_END being moved outside these labels. However, be aware that this may not always be sufficient to prevent gcc(1) from generating non-restartable code within the RAS due to register clobbers. It is, therefore, strongly recommended that restartable atomic sequences are coded in assembly. RAS blocks within assembly code can be specified by using the following macros:

RAS_START_ASM(name): Similar to RAS_START but for use in assembly source code.
RAS_END_ASM(name): Similar to RAS_END but for use in assembly source code.
RAS_START_ASM_HIDDEN(name): Similar to RAS_START_ASM except that the symbol will not be placed in the dynamic symbol table.
RAS_END_ASM_HIDDEN(name): Similar to RAS_END_ASM except that the symbol will not be placed in the dynamic symbol table.