NAME
rasctl —
restartable atomic sequences
LIBRARY
library “libc”
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.
SEE ALSO
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 theRAS. 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
RASand is used to create the first argument torasctl. RAS_SIZE(name)- Returns the size of a
RASand is used as second argument torasctl.
-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_STARTbut for use in assembly source code. RAS_END_ASM(name)- Similar to
RAS_ENDbut for use in assembly source code. RAS_START_ASM_HIDDEN(name)- Similar to
RAS_START_ASMexcept that the symbol will not be placed in the dynamic symbol table. RAS_END_ASM_HIDDEN(name)- Similar to
RAS_END_ASMexcept that the symbol will not be placed in the dynamic symbol table.