Re: [PATCH v15] sys_membarrier(): system-wide memory barrier (generic, x86)

[Mathieu Desnoyers]

Following advice from Peter Zijlstra, I'll send an updated v16 re-adding
the flags argument, even if unused, since it is a rule for all new
system calls to have it. Meanwhile, here is a man page for sys_membarrier.
(attached)

Thanks,

Mathieu

----- Original Message -----
> [ Not in this patch: tests (Pranith Kumar has a patch for this), man page. ]
> 
> Here is an implementation of a new system call, sys_membarrier(), which
> executes a memory barrier on all threads running on the system. It is
> implemented by calling synchronize_sched(). It can be used to distribute
> the cost of user-space memory barriers asymmetrically by transforming
> pairs of memory barriers into pairs consisting of sys_membarrier() and a
> compiler barrier. For synchronization primitives that distinguish
> between read-side and write-side (e.g. userspace RCU [1], rwlocks), the
> read-side can be accelerated significantly by moving the bulk of the
> memory barrier overhead to the write-side.
> 
> It is based on kernel v4.0.
> 
> To explain the benefit of this scheme, let's introduce two example threads:
> 
> Thread A (non-frequent, e.g. executing liburcu synchronize_rcu())
> Thread B (frequent, e.g. executing liburcu
> rcu_read_lock()/rcu_read_unlock())
> 
> In a scheme where all smp_mb() in thread A are ordering memory accesses
> with respect to smp_mb() present in Thread B, we can change each
> smp_mb() within Thread A into calls to sys_membarrier() and each
> smp_mb() within Thread B into compiler barriers "barrier()".
> 
> Before the change, we had, for each smp_mb() pairs:
> 
> Thread A                    Thread B
> previous mem accesses       previous mem accesses
> smp_mb()                    smp_mb()
> following mem accesses      following mem accesses
> 
> After the change, these pairs become:
> 
> Thread A                    Thread B
> prev mem accesses           prev mem accesses
> sys_membarrier()            barrier()
> follow mem accesses         follow mem accesses
> 
> As we can see, there are two possible scenarios: either Thread B memory
> accesses do not happen concurrently with Thread A accesses (1), or they
> do (2).
> 
> 1) Non-concurrent Thread A vs Thread B accesses:
> 
> Thread A                    Thread B
> prev mem accesses
> sys_membarrier()
> follow mem accesses
>                             prev mem accesses
>                             barrier()
>                             follow mem accesses
> 
> In this case, thread B accesses will be weakly ordered. This is OK,
> because at that point, thread A is not particularly interested in
> ordering them with respect to its own accesses.
> 
> 2) Concurrent Thread A vs Thread B accesses
> 
> Thread A                    Thread B
> prev mem accesses           prev mem accesses
> sys_membarrier()            barrier()
> follow mem accesses         follow mem accesses
> 
> In this case, thread B accesses, which are ensured to be in program
> order thanks to the compiler barrier, will be "upgraded" to full
> smp_mb() by synchronize_sched().
> 
> * Benchmarks
> 
> On Intel Xeon E5405 (8 cores)
> (one thread is calling sys_membarrier, the other 7 threads are busy
> looping)
> 
> 1000 non-expedited sys_membarrier calls in 33s = 33 milliseconds/call.
> 
> * User-space user of this system call: Userspace RCU library
> 
> Both the signal-based and the sys_membarrier userspace RCU schemes
> permit us to remove the memory barrier from the userspace RCU
> rcu_read_lock() and rcu_read_unlock() primitives, thus significantly
> accelerating them. These memory barriers are replaced by compiler
> barriers on the read-side, and all matching memory barriers on the
> write-side are turned into an invocation of a memory barrier on all
> active threads in the process. By letting the kernel perform this
> synchronization rather than dumbly sending a signal to every process
> threads (as we currently do), we diminish the number of unnecessary wake
> ups and only issue the memory barriers on active threads. Non-running
> threads do not need to execute such barrier anyway, because these are
> implied by the scheduler context switches.
> 
> Results in liburcu:
> 
> Operations in 10s, 6 readers, 2 writers:
> 
> memory barriers in reader:    1701557485 reads, 3129842 writes
> signal-based scheme:          9825306874 reads,    5386 writes
> sys_membarrier:               7992076602 reads,     220 writes
> 
> The dynamic sys_membarrier availability check adds some overhead to
> the read-side compared to the signal-based scheme, but besides that,
> with the expedited scheme, we can see that we are close to the read-side
> performance of the signal-based scheme. However, this non-expedited
> sys_membarrier implementation has a much slower grace period than signal
> and memory barrier schemes.
> 
> An expedited version of this system call can be added later on to speed
> up the grace period. Its implementation will likely depend on reading
> the cpu_curr()->mm without holding each CPU's rq lock.
> 
> This patch adds the system call to x86 and to asm-generic.
> 
> [1] http://urcu.so
> 
> Changes since v14:
> - Take care of Thomas Gleixner's comments.
> 
> Changes since v13:
> - Move to kernel/membarrier.c.
> - Remove MEMBARRIER_PRIVATE flag.
> - Add MAINTAINERS file entry.
> 
> Changes since v12:
> - Remove _FLAG suffix from uapi flags.
> - Add Expert menuconfig option CONFIG_MEMBARRIER (default=y).
> - Remove EXPEDITED mode. Only implement non-expedited for now, until
>   reading the cpu_curr()->mm can be done without holding the CPU's rq
>   lock.
> 
> Changes since v11:
> - 5 years have passed.
> - Rebase on v3.19 kernel.
> - Add futex-alike PRIVATE vs SHARED semantic: private for per-process
>   barriers, non-private for memory mappings shared between processes.
> - Simplify user API.
> - Code refactoring.
> 
> Changes since v10:
> - Apply Randy's comments.
> - Rebase on 2.6.34-rc4 -tip.
> 
> Changes since v9:
> - Clean up #ifdef CONFIG_SMP.
> 
> Changes since v8:
> - Go back to rq spin locks taken by sys_membarrier() rather than adding
>   memory barriers to the scheduler. It implies a potential RoS
>   (reduction of service) if sys_membarrier() is executed in a busy-loop
>   by a user, but nothing more than what is already possible with other
>   existing system calls, but saves memory barriers in the scheduler fast
>   path.
> - re-add the memory barrier comments to x86 switch_mm() as an example to
>   other architectures.
> - Update documentation of the memory barriers in sys_membarrier and
>   switch_mm().
> - Append execution scenarios to the changelog showing the purpose of
>   each memory barrier.
> 
> Changes since v7:
> - Move spinlock-mb and scheduler related changes to separate patches.
> - Add support for sys_membarrier on x86_32.
> - Only x86 32/64 system calls are reserved in this patch. It is planned
>   to incrementally reserve syscall IDs on other architectures as these
>   are tested.
> 
> Changes since v6:
> - Remove some unlikely() not so unlikely.
> - Add the proper scheduler memory barriers needed to only use the RCU
>   read lock in sys_membarrier rather than take each runqueue spinlock:
> - Move memory barriers from per-architecture switch_mm() to schedule()
>   and finish_lock_switch(), where they clearly document that all data
>   protected by the rq lock is guaranteed to have memory barriers issued
>   between the scheduler update and the task execution. Replacing the
>   spin lock acquire/release barriers with these memory barriers imply
>   either no overhead (x86 spinlock atomic instruction already implies a
>   full mb) or some hopefully small overhead caused by the upgrade of the
>   spinlock acquire/release barriers to more heavyweight smp_mb().
> - The "generic" version of spinlock-mb.h declares both a mapping to
>   standard spinlocks and full memory barriers. Each architecture can
>   specialize this header following their own need and declare
>   CONFIG_HAVE_SPINLOCK_MB to use their own spinlock-mb.h.
> - Note: benchmarks of scheduler overhead with specialized spinlock-mb.h
>   implementations on a wide range of architecture would be welcome.
> 
> Changes since v5:
> - Plan ahead for extensibility by introducing mandatory/optional masks
>   to the "flags" system call parameter. Past experience with accept4(),
>   signalfd4(), eventfd2(), epoll_create1(), dup3(), pipe2(), and
>   inotify_init1() indicates that this is the kind of thing we want to
>   plan for. Return -EINVAL if the mandatory flags received are unknown.
> - Create include/linux/membarrier.h to define these flags.
> - Add MEMBARRIER_QUERY optional flag.
> 
> Changes since v4:
> - Add "int expedited" parameter, use synchronize_sched() in the
>   non-expedited case. Thanks to Lai Jiangshan for making us consider
>   seriously using synchronize_sched() to provide the low-overhead
>   membarrier scheme.
> - Check num_online_cpus() == 1, quickly return without doing nothing.
> 
> Changes since v3a:
> - Confirm that each CPU indeed runs the current task's ->mm before
>   sending an IPI. Ensures that we do not disturb RT tasks in the
>   presence of lazy TLB shootdown.
> - Document memory barriers needed in switch_mm().
> - Surround helper functions with #ifdef CONFIG_SMP.
> 
> Changes since v2:
> - simply send-to-many to the mm_cpumask. It contains the list of
>   processors we have to IPI to (which use the mm), and this mask is
>   updated atomically.
> 
> Changes since v1:
> - Only perform the IPI in CONFIG_SMP.
> - Only perform the IPI if the process has more than one thread.
> - Only send IPIs to CPUs involved with threads belonging to our process.
> - Adaptative IPI scheme (single vs many IPI with threshold).
> - Issue smp_mb() at the beginning and end of the system call.
> 
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> CC: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> CC: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
> CC: Steven Rostedt <rostedt@xxxxxxxxxxx>
> CC: Nicholas Miell <nmiell@xxxxxxxxxxx>
> CC: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> CC: Ingo Molnar <mingo@xxxxxxxxxx>
> CC: Alan Cox <gnomes@xxxxxxxxxxxxxxxxxxx>
> CC: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>
> CC: Stephen Hemminger <stephen@xxxxxxxxxxxxxxxxxx>
> CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> CC: David Howells <dhowells@xxxxxxxxxx>
> CC: "Pranith Kumar" <bobby.prani@xxxxxxxxx>
> CC: Michael Kerrisk <mtk.manpages@xxxxxxxxx>
> ---
>  MAINTAINERS                       |    8 ++++
>  arch/x86/syscalls/syscall_32.tbl  |    1 +
>  arch/x86/syscalls/syscall_64.tbl  |    1 +
>  include/linux/syscalls.h          |    2 +
>  include/uapi/asm-generic/unistd.h |    4 ++-
>  include/uapi/linux/Kbuild         |    1 +
>  include/uapi/linux/membarrier.h   |   65
>  +++++++++++++++++++++++++++++++++++++
>  init/Kconfig                      |   13 +++++++
>  kernel/Makefile                   |    1 +
>  kernel/membarrier.c               |   58 +++++++++++++++++++++++++++++++++
>  kernel/sys_ni.c                   |    3 ++
>  11 files changed, 156 insertions(+), 1 deletions(-)
>  create mode 100644 include/uapi/linux/membarrier.h
>  create mode 100644 kernel/membarrier.c
> 
> diff --git a/MAINTAINERS b/MAINTAINERS
> index efbcb50..a94cc7b 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -6328,6 +6328,14 @@ W:	http://www.mellanox.com
>  Q:	http://patchwork.ozlabs.org/project/netdev/list/
>  F:	drivers/net/ethernet/mellanox/mlx4/en_*
>  
> +MEMBARRIER SUPPORT
> +M:	Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> +M:	"Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
> +L:	linux-kernel@xxxxxxxxxxxxxxx
> +S:	Supported
> +F:	kernel/membarrier.c
> +F:	include/uapi/linux/membarrier.h
> +
>  MEMORY MANAGEMENT
>  L:	linux-mm@xxxxxxxxx
>  W:	http://www.linux-mm.org
> diff --git a/arch/x86/syscalls/syscall_32.tbl
> b/arch/x86/syscalls/syscall_32.tbl
> index b3560ec..439415f 100644
> --- a/arch/x86/syscalls/syscall_32.tbl
> +++ b/arch/x86/syscalls/syscall_32.tbl
> @@ -365,3 +365,4 @@
>  356	i386	memfd_create		sys_memfd_create
>  357	i386	bpf			sys_bpf
>  358	i386	execveat		sys_execveat			stub32_execveat
> +359	i386	membarrier		sys_membarrier
> diff --git a/arch/x86/syscalls/syscall_64.tbl
> b/arch/x86/syscalls/syscall_64.tbl
> index 8d656fb..823130d 100644
> --- a/arch/x86/syscalls/syscall_64.tbl
> +++ b/arch/x86/syscalls/syscall_64.tbl
> @@ -329,6 +329,7 @@
>  320	common	kexec_file_load		sys_kexec_file_load
>  321	common	bpf			sys_bpf
>  322	64	execveat		stub_execveat
> +323	common	membarrier		sys_membarrier
>  
>  #
>  # x32-specific system call numbers start at 512 to avoid cache impact
> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> index 76d1e38..772be39 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -884,4 +884,6 @@ asmlinkage long sys_execveat(int dfd, const char __user
> *filename,
>  			const char __user *const __user *argv,
>  			const char __user *const __user *envp, int flags);
>  
> +asmlinkage long sys_membarrier(int cmd);
> +
>  #endif
> diff --git a/include/uapi/asm-generic/unistd.h
> b/include/uapi/asm-generic/unistd.h
> index e016bd9..8da542a 100644
> --- a/include/uapi/asm-generic/unistd.h
> +++ b/include/uapi/asm-generic/unistd.h
> @@ -709,9 +709,11 @@ __SYSCALL(__NR_memfd_create, sys_memfd_create)
>  __SYSCALL(__NR_bpf, sys_bpf)
>  #define __NR_execveat 281
>  __SC_COMP(__NR_execveat, sys_execveat, compat_sys_execveat)
> +#define __NR_membarrier 282
> +__SYSCALL(__NR_membarrier, sys_membarrier)
>  
>  #undef __NR_syscalls
> -#define __NR_syscalls 282
> +#define __NR_syscalls 283
>  
>  /*
>   * All syscalls below here should go away really,
> diff --git a/include/uapi/linux/Kbuild b/include/uapi/linux/Kbuild
> index 68ceb97..4684a8d 100644
> --- a/include/uapi/linux/Kbuild
> +++ b/include/uapi/linux/Kbuild
> @@ -249,6 +249,7 @@ header-y += mdio.h
>  header-y += media.h
>  header-y += media-bus-format.h
>  header-y += mei.h
> +header-y += membarrier.h
>  header-y += memfd.h
>  header-y += mempolicy.h
>  header-y += meye.h
> diff --git a/include/uapi/linux/membarrier.h
> b/include/uapi/linux/membarrier.h
> new file mode 100644
> index 0000000..c532d8f
> --- /dev/null
> +++ b/include/uapi/linux/membarrier.h
> @@ -0,0 +1,65 @@
> +#ifndef _UAPI_LINUX_MEMBARRIER_H
> +#define _UAPI_LINUX_MEMBARRIER_H
> +
> +/*
> + * linux/membarrier.h
> + *
> + * membarrier system call API
> + *
> + * Copyright (c) 2010, 2015 Mathieu Desnoyers
> <mathieu.desnoyers@xxxxxxxxxxxx>
> + *
> + * Permission is hereby granted, free of charge, to any person obtaining a
> copy
> + * of this software and associated documentation files (the "Software"), to
> deal
> + * in the Software without restriction, including without limitation the
> rights
> + * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
> + * copies of the Software, and to permit persons to whom the Software is
> + * furnished to do so, subject to the following conditions:
> + *
> + * The above copyright notice and this permission notice shall be included
> in
> + * all copies or substantial portions of the Software.
> + *
> + * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
> OR
> + * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
> + * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
> THE
> + * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
> + * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
> FROM,
> + * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
> THE
> + * SOFTWARE.
> + */
> +
> +/*
> + * All memory accesses performed in program order from each thread on
> + * the system is guaranteed to be ordered with respect to sys_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 pair of barrier(), sys_membarrier() and
> + * smp_mb() :
> + *
> + * The pair ordering is detailed as (O: ordered, X: not ordered):
> + *
> + *                        barrier()   smp_mb() sys_membarrier()
> + *        barrier()          X           X            O
> + *        smp_mb()           X           O            O
> + *        sys_membarrier()   O           O            O
> + */
> +
> +/**
> + * enum membarrier_cmd - membarrier system call command
> + *
> + * Command to be passed to the membarrier system call.
> + */
> +enum membarrier_cmd {
> +	/*
> +	 * Query the set of supported commands. It returns a bitmask of valid
> +	 * commands.
> +	 */
> +	MEMBARRIER_CMD_QUERY = 0,
> +	/*
> +	 * Execute a memory barrier on all running threads.
> +	 */
> +	MEMBARRIER_CMD_SHARED = (1 << 0),
> +};
> +
> +#endif /* _UAPI_LINUX_MEMBARRIER_H */
> diff --git a/init/Kconfig b/init/Kconfig
> index f5dbc6d..89bad6a 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1559,6 +1559,19 @@ config PCI_QUIRKS
>  	  bugs/quirks. Disable this only if your target machine is
>  	  unaffected by PCI quirks.
>  
> +config MEMBARRIER
> +	bool "Enable membarrier() system call" if EXPERT
> +	default y
> +	depends on SMP
> +	help
> +	  Enable the membarrier() system call that allows issuing memory
> +	  barriers across all running threads, which can be used to distribute
> +	  the cost of user-space memory barriers asymmetrically by transforming
> +	  pairs of memory barriers into pairs consisting of membarrier() and a
> +	  compiler barrier.
> +
> +	  If unsure, say Y.
> +
>  config EMBEDDED
>  	bool "Embedded system"
>  	option allnoconfig_y
> diff --git a/kernel/Makefile b/kernel/Makefile
> index 1408b33..1cafa11 100644
> --- a/kernel/Makefile
> +++ b/kernel/Makefile
> @@ -96,6 +96,7 @@ obj-$(CONFIG_CRASH_DUMP) += crash_dump.o
>  obj-$(CONFIG_JUMP_LABEL) += jump_label.o
>  obj-$(CONFIG_CONTEXT_TRACKING) += context_tracking.o
>  obj-$(CONFIG_TORTURE_TEST) += torture.o
> +obj-$(CONFIG_MEMBARRIER) += membarrier.o
>  
>  $(obj)/configs.o: $(obj)/config_data.h
>  
> diff --git a/kernel/membarrier.c b/kernel/membarrier.c
> new file mode 100644
> index 0000000..84f2da6
> --- /dev/null
> +++ b/kernel/membarrier.c
> @@ -0,0 +1,58 @@
> +/*
> + * Copyright (C) 2010, 2015 Mathieu Desnoyers
> <mathieu.desnoyers@xxxxxxxxxxxx>
> + *
> + * membarrier system call
> + *
> + * This program is free software; you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License as published by
> + * the Free Software Foundation; either version 2 of the License, or
> + * (at your option) any later version.
> + *
> + * This program is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> + * GNU General Public License for more details.
> + */
> +
> +#include <linux/syscalls.h>
> +#include <linux/membarrier.h>
> +
> +/*
> + * Bitmask made from a "or" of all commands within enum membarrier_cmd,
> + * except MEMBARRIER_CMD_QUERY.
> + */
> +#define MEMBARRIER_CMD_BITMASK	(MEMBARRIER_CMD_SHARED)
> +
> +/**
> + * sys_membarrier - issue memory barriers on all running threads
> + * @cmd:   MEMBARRIER_CMD_QUERY:
> + *             Query the set of supported commands. It returns a bitmask of
> + *             supported commands.
> + *         MEMBARRIER_CMD_SHARED:
> + *             Execute a memory barrier on all running threads. Upon
> + *             return from system call, the caller thread is ensured that
> + *             all running threads 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 threads from all processes running on the system.
> + *             This command returns 0.
> + *
> + * If this system call is not implemented, -ENOSYS is returned. If the
> + * command specified does not exist, or if the command argument is invalid,
> + * this system call returns -EINVAL. For a given command on a given kernel,
> + * this system call will always return the same value.
> + */
> +SYSCALL_DEFINE1(membarrier, int, cmd)
> +{
> +	switch (cmd) {
> +	case MEMBARRIER_CMD_QUERY:
> +		return MEMBARRIER_CMD_BITMASK;
> +	case MEMBARRIER_CMD_SHARED:
> +		if (num_online_cpus() > 1)
> +			synchronize_sched();
> +		return 0;
> +	default:
> +		return -EINVAL;
> +	}
> +}
> diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
> index 5adcb0a..5913b84 100644
> --- a/kernel/sys_ni.c
> +++ b/kernel/sys_ni.c
> @@ -229,3 +229,6 @@ cond_syscall(sys_bpf);
>  
>  /* execveat */
>  cond_syscall(sys_execveat);
> +
> +/* membarrier */
> +cond_syscall(sys_membarrier);
> --
> 1.7.7.3
> 
> 

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
.\" Copyright 2015 Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
.\"
.\" %%%LICENSE_START(VERBATIM)
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.\"
.TH MEMBARRIER 2 2015-04-15 "Linux" "Linux Programmer's Manual"
.SH NAME
membarrier \- issue memory barriers on a set of threads
.SH SYNOPSIS
.B #include <linux/membarrier.h>
.sp
.BI "int membarrier(int " cmd ", int " flags ");
.sp
.SH DESCRIPTION
The
.I cmd
argument is one of the following:

.TP
.B MEMBARRIER_CMD_QUERY
Query the set of supported commands. It returns a bitmask of supported
commands.
.TP
.B MEMBARRIER_CMD_SHARED
Execute a memory barrier on all threads running on the system. Upon
return from system call, the caller thread is ensured that all running
threads 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 threads from all processes running on the system.
This command returns 0.

.PP
The
.I flags
argument is currently unused.

.PP
All memory accesses performed in program order from each targeted thread
is guaranteed to be ordered with respect to sys_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 pair of barrier(), sys_membarrier() and smp_mb():

The pair ordering is detailed as (O: ordered, X: not ordered):

                       barrier()   smp_mb() sys_membarrier()
       barrier()          X           X            O
       smp_mb()           X           O            O
       sys_membarrier()   O           O            O

.SH RETURN VALUE
On success, these system calls return zero.
On error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
.TP
.B ENOSYS
System call is not implemented.
.TP
.B EINVAL
Invalid arguments.