RASCTL(2) BSD System Calls Manual RASCTL(2)NAMErasctl — 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 opera‐
tion, 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 ALSOptrace(2)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 identi‐
fies 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 pre‐
vent 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 sym‐
bol 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 ta‐
ble.
BSD April 29, 2008 BSD
