Hi Michael, Please find the membarrier man groff file attached. I re-integrated some changes that went in initially only in the changelog text version back onto this groff source. Please let me know if you find any issue with it. Mathieu ----- On Dec 4, 2015, at 4:44 PM, Michael Kerrisk mtk.manpages@gmail.com wrote: > Hi Mathieu, > > In the patch below you have a man page type of text. Is that > just plain text, or do you have some groff source somewhere? > > Thanks, > > Michael > > > On 07/10/2015 10:58 PM, 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. >> >> The existing applications of which I am aware that would be improved by this >> system call are as follows: >> >> * Through Userspace RCU library (http://urcu.so) >> - DNS server (Knot DNS) https://www.knot-dns.cz/ >> - Network sniffer (http://netsniff-ng.org/) >> - Distributed object storage (https://sheepdog.github.io/sheepdog/) >> - User-space tracing (http://lttng.org) >> - Network storage system (https://www.gluster.org/) >> - Virtual routers >> (https://events.linuxfoundation.org/sites/events/files/slides/DPDK_RCU_0MQ.pdf) >> - Financial software (https://lkml.org/lkml/2015/3/23/189) >> >> Those projects use RCU in userspace to increase read-side speed and >> scalability compared to locking. Especially in the case of RCU used >> by libraries, sys_membarrier can speed up the read-side by moving the >> bulk of the memory barrier cost to synchronize_rcu(). >> >> * Direct users of sys_membarrier >> - core dotnet garbage collector (https://github.com/dotnet/coreclr/issues/198) >> >> Microsoft core dotnet GC developers are planning to use the mprotect() >> side-effect of issuing memory barriers through IPIs as a way to implement >> Windows FlushProcessWriteBuffers() on Linux. They are referring to >> sys_membarrier in their github thread, specifically stating that >> sys_membarrier() is what they are looking for. >> >> This implementation is based on kernel v4.1-rc8. >> >> 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. >> >> [1] http://urcu.so >> >> Signed-off-by: Mathieu Desnoyers >> Reviewed-by: Paul E. McKenney >> Reviewed-by: Josh Triplett >> CC: KOSAKI Motohiro >> CC: Steven Rostedt >> CC: Nicholas Miell >> CC: Linus Torvalds >> CC: Ingo Molnar >> CC: Alan Cox >> CC: Lai Jiangshan >> CC: Stephen Hemminger >> CC: Andrew Morton >> CC: Thomas Gleixner >> CC: Peter Zijlstra >> CC: David Howells >> CC: Pranith Kumar >> CC: Michael Kerrisk >> CC: linux-api@vger.kernel.org >> >> --- >> >> 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 >> >> 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 pro‐ >> 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 ------------------- >> >> Changes since v18: >> - Add unlikely() check to flags, >> - Describe current users in changelog. >> >> 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. >> --- >> MAINTAINERS | 8 +++++ >> arch/x86/entry/syscalls/syscall_32.tbl | 1 + >> arch/x86/entry/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 deletion(-) >> create mode 100644 include/uapi/linux/membarrier.h >> create mode 100644 kernel/membarrier.c >> >> diff --git a/MAINTAINERS b/MAINTAINERS >> index 0d70760..b560da6 100644 >> --- a/MAINTAINERS >> +++ b/MAINTAINERS >> @@ -6642,6 +6642,14 @@ 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 >> +M: "Paul E. McKenney" >> +L: linux-kernel@vger.kernel.org >> +S: Supported >> +F: kernel/membarrier.c >> +F: include/uapi/linux/membarrier.h >> + >> MEMORY MANAGEMENT >> L: linux-mm@kvack.org >> W: http://www.linux-mm.org >> diff --git a/arch/x86/entry/syscalls/syscall_32.tbl >> b/arch/x86/entry/syscalls/syscall_32.tbl >> index ef8187f..e63ad61 100644 >> --- a/arch/x86/entry/syscalls/syscall_32.tbl >> +++ b/arch/x86/entry/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/entry/syscalls/syscall_64.tbl >> b/arch/x86/entry/syscalls/syscall_64.tbl >> index 9ef32d5..87f3cd6 100644 >> --- a/arch/x86/entry/syscalls/syscall_64.tbl >> +++ b/arch/x86/entry/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 b45c45b..d4ab99b 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 1ff9942..e6f229a 100644 >> --- a/include/uapi/linux/Kbuild >> +++ b/include/uapi/linux/Kbuild >> @@ -251,6 +251,7 @@ 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 >> + * >> + * 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 af09b4f..4bba60f 100644 >> --- a/init/Kconfig >> +++ b/init/Kconfig >> @@ -1577,6 +1577,18 @@ 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 43c4c92..92a481b 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..536c727 >> --- /dev/null >> +++ b/kernel/membarrier.c >> @@ -0,0 +1,66 @@ >> +/* >> + * Copyright (C) 2010, 2015 Mathieu Desnoyers >> + * >> + * 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 >> +#include >> + >> +/* >> + * 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 (unlikely(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); >> > > > -- > Michael Kerrisk > Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ > Linux/UNIX System Programming Training: http://man7.org/training/ -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com