MEMBARRIER(2) Linux Programmer's Manual MEMBARRIER(2)
NAME
membarrier - issue memory barriers on a set of threads
SYNOPSIS
#include <linux/membarrier.h>
int membarrier(int cmd, int flags);
DESCRIPTION
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 mem-
ory 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 match-
ing barriers, the fast-side memory barriers by simple compiler barri-
ers, 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 membar-
rier() 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.
MEMBARRIER_CMD_PRIVATE_EXPEDITED (since Linux 4.14)
Execute a memory barrier on each running thread belonging to the
same process as the current thread. Upon return from system
call, the calling thread is assured that all its running threads
siblings have passed through a state where all memory accesses
to user-space addresses match program order between entry to and
return from the system call (non-running threads are de facto in
such a state). This covers only threads from the same process
as the calling thread.
The "expedited" commands complete faster than the non-expedited
ones; they never block, but have the downside of causing extra
overhead. A process needs to register its intent to use the
private expedited command prior to using it.
MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED (since Linux 4.14)
Register the process's intent to use MEMBARRIER_CMD_PRI-
VATE_EXPEDITED.
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 forc-
ing memory accesses to be performed in program order across the bar-
rier, and smp_mb() to represent explicit memory barriers forcing full
memory ordering across the barrier, we have the following ordering ta-
ble 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
RETURN VALUE
On success, the MEMBARRIER_CMD_QUERY operation returns a bit mask of
supported commands, and the MEMBARRIER_CMD_SHARED , MEMBARRIER_CMD_PRI-
VATE_EXPEDITED , and MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED , opera-
tions return zero. On error, -1 is returned, and errno is set appro-
priately.
For a given command, with flags set to 0, this system call is guaran-
teed 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 membar-
rier().
ERRORS
EINVAL cmd is invalid, or flags is nonzero, or the MEMBAR-
RIER_CMD_SHARED command is disabled because the nohz_full CPU
parameter has been set.
ENOSYS The membarrier() system call is not implemented by this kernel.
EPERM The current process was not registered prior to using private
expedited commands.
VERSIONS
The membarrier() system call was added in Linux 4.3.
CONFORMING TO
membarrier() is Linux-specific.
NOTES
A memory barrier instruction is part of the instruction set of archi-
tectures 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.
EXAMPLE
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(int *read_b)
{
a = 1;
asm volatile ("mfence" : : : "memory");
*read_b = b;
}
static void
slow_path(int *read_a)
{
b = 1;
asm volatile ("mfence" : : : "memory");
*read_a = a;
}
int
main(int argc, char **argv)
{
int read_a, read_b;
/*
* Real applications would call fast_path() and slow_path()
* from different threads. Call those from main() to keep
* this example short.
*/
slow_path(&read_a);
fast_path(&read_b);
/*
* read_b == 0 implies read_a == 1 and
* read_a == 0 implies read_b == 1.
*/
if (read_b == 0 && read_a == 0)
abort();
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(int *read_b)
{
a = 1;
asm volatile ("" : : : "memory");
*read_b = b;
}
static void
slow_path(int *read_a)
{
b = 1;
membarrier(MEMBARRIER_CMD_SHARED, 0);
*read_a = a;
}
int
main(int argc, char **argv)
{
int read_a, read_b;
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(&read_a);
fast_path(&read_b);
/*
* read_b == 0 implies read_a == 1 and
* read_a == 0 implies read_b == 1.
*/
if (read_b == 0 && read_a == 0)
abort();
exit(EXIT_SUCCESS);
}
COLOPHON
This page is part of release 4.15 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 2017-11-15 MEMBARRIER(2)
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