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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLE | COLOPHON |
MEMBARRIER(2) Linux Programmer's Manual MEMBARRIER(2)
membarrier - issue memory barriers on a set of threads
#include <linux/membarrier.h>
int membarrier(int cmd, int flags);
The membarrier() system call helps reducing the overhead of the
memory barrier instructions required to order memory accesses on
multi-core systems. However, this system call is heavier than a
memory barrier, so using it effectively is not as simple as replacing
memory barriers with this system call, but requires understanding of
the details below.
Use of memory barriers needs to be done taking into account that a
memory barrier always needs to be either matched with its memory
barrier counterparts, or that the architecture's memory model doesn't
require the matching barriers.
There are cases where one side of the matching barriers (which we
will refer to as "fast side") is executed much more often than the
other (which we will refer to as "slow side"). This is a prime
target for the use of membarrier(). The key idea is to replace, for
these matching barriers, the fast-side memory barriers by simple
compiler barriers, for example:
asm volatile ("" : : : "memory")
and replace the slow-side memory barriers by calls to membarrier().
This will add overhead to the slow side, and remove overhead from the
fast side, thus resulting in an overall performance increase as long
as the slow side is infrequent enough that the overhead of the
membarrier() calls does not outweigh the performance gain on the fast
side.
The cmd argument is one of the following:
MEMBARRIER_CMD_QUERY
Query the set of supported commands. The return value of the
call is a bit mask of supported commands.
MEMBARRIER_CMD_QUERY, which has the value 0, is not itself
included in this bit mask. This command is always supported
(on kernels where membarrier() is provided).
MEMBARRIER_CMD_SHARED
Ensure that all threads from all processes on the system pass
through a state where all memory accesses to user-space
addresses match program order between entry to and return from
the membarrier() system call. All threads on the system are
targeted by this command.
The flags argument is currently unused and must be specified as 0.
All memory accesses performed in program order from each targeted
thread are guaranteed to be ordered with respect to membarrier().
If we use the semantic barrier() to represent a compiler barrier
forcing memory accesses to be performed in program order across the
barrier, and smp_mb() to represent explicit memory barriers forcing
full memory ordering across the barrier, we have the following
ordering table for each pairing of barrier(), membarrier() and
smp_mb(). The pair ordering is detailed as (O: ordered, X: not
ordered):
barrier() smp_mb() membarrier()
barrier() X X O
smp_mb() X O O
membarrier() O O O
On success, the MEMBARRIER_CMD_QUERY operation returns a bit mask of
supported commands and the MEMBARRIER_CMD_SHARED operation returns
zero. On error, -1 is returned, and errno is set appropriately.
For a given command, with flags set to 0, this system call is
guaranteed to always return the same value until reboot. Further
calls with the same arguments will lead to the same result.
Therefore, with flags set to 0, error handling is required only for
the first call to membarrier().
EINVAL cmd is invalid or flags is non-zero.
ENOSYS The membarrier() system call is not implemented by this
kernel.
ENOSYS (since Linux 4.11)
The membarrier() system call is disabled because the nohz_full
CPU parameter has been set.
The membarrier() system call was added in Linux 4.3.
membarrier() is Linux-specific.
A memory barrier instruction is part of the instruction set of
architectures with weakly-ordered memory models. It orders memory
accesses prior to the barrier and after the barrier with respect to
matching barriers on other cores. For instance, a load fence can
order loads prior to and following that fence with respect to stores
ordered by store fences.
Program order is the order in which instructions are ordered in the
program assembly code.
Examples where membarrier() can be useful include implementations of
Read-Copy-Update libraries and garbage collectors.
Assuming a multithreaded application where "fast_path()" is executed
very frequently, and where "slow_path()" is executed infrequently,
the following code (x86) can be transformed using membarrier():
#include <stdlib.h>
static volatile int a, b;
static void
fast_path(void)
{
int read_a, read_b;
read_b = b;
asm volatile ("mfence" : : : "memory");
read_a = a;
/* read_b == 1 implies read_a == 1. */
if (read_b == 1 && read_a == 0)
abort();
}
static void
slow_path(void)
{
a = 1;
asm volatile ("mfence" : : : "memory");
b = 1;
}
int
main(int argc, char **argv)
{
/*
* Real applications would call fast_path() and slow_path()
* from different threads. Call those from main() to keep
* this example short.
*/
slow_path();
fast_path();
exit(EXIT_SUCCESS);
}
The code above transformed to use membarrier() becomes:
#define _GNU_SOURCE
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/syscall.h>
#include <linux/membarrier.h>
static volatile int a, b;
static int
membarrier(int cmd, int flags)
{
return syscall(__NR_membarrier, cmd, flags);
}
static int
init_membarrier(void)
{
int ret;
/* Check that membarrier() is supported. */
ret = membarrier(MEMBARRIER_CMD_QUERY, 0);
if (ret < 0) {
perror("membarrier");
return -1;
}
if (!(ret & MEMBARRIER_CMD_SHARED)) {
fprintf(stderr,
"membarrier does not support MEMBARRIER_CMD_SHARED\n");
return -1;
}
return 0;
}
static void
fast_path(void)
{
int read_a, read_b;
read_b = b;
asm volatile ("" : : : "memory");
read_a = a;
/* read_b == 1 implies read_a == 1. */
if (read_b == 1 && read_a == 0)
abort();
}
static void
slow_path(void)
{
a = 1;
membarrier(MEMBARRIER_CMD_SHARED, 0);
b = 1;
}
int
main(int argc, char **argv)
{
if (init_membarrier())
exit(EXIT_FAILURE);
/*
* Real applications would call fast_path() and slow_path()
* from different threads. Call those from main() to keep
* this example short.
*/
slow_path();
fast_path();
exit(EXIT_SUCCESS);
}
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-07-13 MEMBARRIER(2)
Pages that refer to this page: syscalls(2)