Re: [PATCH v18 for v4.1-rc2 1/3] sys_membarrier(): system-wide memory barrier (generic, x86)

[josh]

On Wed, May 06, 2015 at 03:21:06PM -0400, Mathieu Desnoyers wrote:
> 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 v4.1-rc2.
> 
> 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, 2202847 writes
> signal-based scheme:          9830061167 reads,    6700 writes
> sys_membarrier:               9952759104 reads,     425 writes
> sys_membarrier (dyn. check):  7970328887 reads,     425 writes
> 
> The dynamic sys_membarrier availability check adds some overhead to
> the read-side compared to the signal-based scheme, but besides that,
> sys_membarrier slightly outperforms the signal-based scheme. However,
> this non-expedited sys_membarrier implementation has a much slower grace
> period than signal and memory barrier schemes.
> 
> Besides diminishing the number of wake-ups, one major advantage of the
> membarrier system call over the signal-based scheme is that it does not
> need to reserve a signal. This plays much more nicely with libraries,
> and with processes injected into for tracing purposes, for which we
> cannot expect that signals will be unused by the application.
> 
> 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.
> 
> membarrier(2) man page:
> --------------- snip -------------------
> 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 cmd argument is one of the following:
> 
>        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 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 proâ
>               cesses running on the system.  This command returns 0.
> 
>        The flags argument needs to be 0. For future extensions.
> 
>        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
> 
> RETURN VALUE
>        On success, these system calls return zero.  On error, -1 is  returned,
>        and errno is set appropriately. For a given command, with flags
>        argument set to 0, this system call is guaranteed to always return the
>        same value until reboot.
> 
> ERRORS
>        ENOSYS System call is not implemented.
> 
>        EINVAL Invalid arguments.
> 
> Linux                             2015-04-15                     MEMBARRIER(2)
> --------------- snip -------------------
> 
> [1] http://urcu.so
> 
> Changes since v17:
> - Update commit message.
> 
> Changes since v16:
> - Update documentation.
> - Add man page to changelog.
> - Build sys_membarrier on !CONFIG_SMP. It allows userspace applications
>   to not care about the number of processors on the system.  Based on
>   recommendations from Stephen Hemminger and Steven Rostedt.
> - Check that flags argument is 0, update documentation to require it.
> 
> Changes since v15:
> - Add flags argument in addition to cmd.
> - Update documentation.
> 
> 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>

Reviewed-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>

But also, the "snip" and "changes since" should not be in the commit message,
while this list of signoffs and CCs should be.

- Josh Triplett

> 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>
> CC: linux-api@xxxxxxxxxxxxxxx
> ---
>  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   |   53 +++++++++++++++++++++++++++++
>  init/Kconfig                      |   12 +++++++
>  kernel/Makefile                   |    1 +
>  kernel/membarrier.c               |   66 +++++++++++++++++++++++++++++++++++++
>  kernel/sys_ni.c                   |    3 ++
>  11 files changed, 151 insertions(+), 1 deletions(-)
>  create mode 100644 include/uapi/linux/membarrier.h
>  create mode 100644 kernel/membarrier.c
> 
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 781e099..fcb63d4 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -6370,6 +6370,14 @@ 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 ef8187f..e63ad61 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 9ef32d5..87f3cd6 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..51a9054 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, int flags);
> +
>  #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 1a0006a..7bcc827 100644
> --- a/include/uapi/linux/Kbuild
> +++ b/include/uapi/linux/Kbuild
> @@ -250,6 +250,7 @@ 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..e0b108b
> --- /dev/null
> +++ b/include/uapi/linux/membarrier.h
> @@ -0,0 +1,53 @@
> +#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.
> + */
> +
> +/**
> + * enum membarrier_cmd - membarrier system call command
> + * @MEMBARRIER_CMD_QUERY:   Query the set of supported commands. It returns
> + *                          a bitmask of valid 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.
> + *
> + * Command to be passed to the membarrier system call. The commands need to
> + * be a single bit each, except for MEMBARRIER_CMD_QUERY which is assigned to
> + * the value 0.
> + */
> +enum membarrier_cmd {
> +	MEMBARRIER_CMD_QUERY = 0,
> +	MEMBARRIER_CMD_SHARED = (1 << 0),
> +};
> +
> +#endif /* _UAPI_LINUX_MEMBARRIER_H */
> diff --git a/init/Kconfig b/init/Kconfig
> index dc24dec..307e406 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1583,6 +1583,18 @@ 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
> +	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 60c302c..05191fd 100644
> --- a/kernel/Makefile
> +++ b/kernel/Makefile
> @@ -98,6 +98,7 @@ 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..a20b279
> --- /dev/null
> +++ b/kernel/membarrier.c
> @@ -0,0 +1,66 @@
> +/*
> + * 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 a set of threads
> + * @cmd:   Takes command values defined in enum membarrier_cmd.
> + * @flags: Currently needs to be 0. For future extensions.
> + *
> + * 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, with flags argument
> + * set to 0, this system call is guaranteed to always return the same value
> + * until reboot.
> + *
> + * 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
> + */
> +SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
> +{
> +	if (flags)
> +		return -EINVAL;
> +	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 7995ef5..eb4fde0 100644
> --- a/kernel/sys_ni.c
> +++ b/kernel/sys_ni.c
> @@ -243,3 +243,6 @@ cond_syscall(sys_bpf);
>  
>  /* execveat */
>  cond_syscall(sys_execveat);
> +
> +/* membarrier */
> +cond_syscall(sys_membarrier);
> -- 
> 1.7.7.3
> 
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