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diff --git a/man2/membarrier.2 b/man2/membarrier.2 new file mode 100644 index 0000000..f118fd0 --- /dev/null +++ b/man2/membarrier.2 @@ -0,0 +1,460 @@ +'\" t +.\" Copyright 2015-2017 Mathieu Desnoyers <mathieu.desnoyers@efficios.com> +.\" +.\" SPDX-License-Identifier: Linux-man-pages-copyleft +.\" +.TH membarrier 2 2023-05-03 "Linux man-pages 6.05.01" +.SH NAME +membarrier \- issue memory barriers on a set of threads +.SH LIBRARY +Standard C library +.RI ( libc ", " \-lc ) +.SH SYNOPSIS +.nf +.PP +.BR "#include <linux/membarrier.h>" \ +" /* Definition of " MEMBARRIER_* " constants */" +.BR "#include <sys/syscall.h>" " /* Definition of " SYS_* " constants */" +.B #include <unistd.h> +.PP +.BI "int syscall(SYS_membarrier, int " cmd ", unsigned int " flags \ +", int " cpu_id ); +.fi +.PP +.IR Note : +glibc provides no wrapper for +.BR membarrier (), +necessitating the use of +.BR syscall (2). +.SH DESCRIPTION +The +.BR 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 +.I not +as simple as replacing memory barriers with this +system call, but requires understanding of the details below. +.PP +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. +.PP +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 +.BR membarrier (). +The key idea is to replace, for these matching +barriers, the fast-side memory barriers by simple compiler barriers, +for example: +.PP +.in +4n +.EX +asm volatile ("" : : : "memory") +.EE +.in +.PP +and replace the slow-side memory barriers by calls to +.BR membarrier (). +.PP +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 +.BR membarrier () +calls does not outweigh the performance gain on the fast side. +.PP +The +.I cmd +argument is one of the following: +.TP +.BR MEMBARRIER_CMD_QUERY " (since Linux 4.3)" +Query the set of supported commands. +The return value of the call is a bit mask of supported +commands. +.BR MEMBARRIER_CMD_QUERY , +which has the value 0, +is not itself included in this bit mask. +This command is always supported (on kernels where +.BR membarrier () +is provided). +.TP +.BR MEMBARRIER_CMD_GLOBAL " (since Linux 4.16)" +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 +.BR membarrier () +system call. +All threads on the system are targeted by this command. +.TP +.BR MEMBARRIER_CMD_GLOBAL_EXPEDITED " (since Linux 4.16)" +Execute a memory barrier on all running threads of all processes that +previously registered with +.BR MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED . +.IP +Upon return from the system call, the calling thread has a guarantee 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 guarantee is provided only for the threads of processes that +previously registered with +.BR MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED . +.IP +Given that registration is about the intent to receive the barriers, it +is valid to invoke +.B MEMBARRIER_CMD_GLOBAL_EXPEDITED +from a process that has not employed +.BR MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED . +.IP +The "expedited" commands complete faster than the non-expedited ones; +they never block, but have the downside of causing extra overhead. +.TP +.BR MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED " (since Linux 4.16)" +Register the process's intent to receive +.B MEMBARRIER_CMD_GLOBAL_EXPEDITED +memory barriers. +.TP +.BR MEMBARRIER_CMD_PRIVATE_EXPEDITED " (since Linux 4.14)" +Execute a memory barrier on each running thread belonging to the same +process as the calling thread. +.IP +Upon return from the system call, the calling +thread has a guarantee that all its running thread 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 guarantee is provided only for threads in +the same process as the calling thread. +.IP +The "expedited" commands complete faster than the non-expedited ones; +they never block, but have the downside of causing extra overhead. +.IP +A process must register its intent to use the private +expedited command prior to using it. +.TP +.BR MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED " (since Linux 4.14)" +Register the process's intent to use +.BR MEMBARRIER_CMD_PRIVATE_EXPEDITED . +.TP +.BR MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE " (since Linux 4.16)" +In addition to providing the memory ordering guarantees described in +.BR MEMBARRIER_CMD_PRIVATE_EXPEDITED , +upon return from system call the calling thread has a guarantee that all its +running thread siblings have executed a core serializing instruction. +This guarantee is provided only for threads in +the same process as the calling thread. +.IP +The "expedited" commands complete faster than the non-expedited ones, +they never block, but have the downside of causing extra overhead. +.IP +A process must register its intent to use the private expedited sync +core command prior to using it. +.TP +.BR MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE " (since Linux 4.16)" +Register the process's intent to use +.BR MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE . +.TP +.BR MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ " (since Linux 5.10)" +Ensure the caller thread, upon return from system call, that all its +running thread siblings have any currently running rseq critical sections +restarted if +.I flags +parameter is 0; if +.I flags +parameter is +.BR MEMBARRIER_CMD_FLAG_CPU , +then this operation is performed only on CPU indicated by +.IR cpu_id . +This guarantee is provided only for threads in +the same process as the calling thread. +.IP +RSEQ membarrier is only available in the "private expedited" form. +.IP +A process must register its intent to use the private expedited rseq +command prior to using it. +.TP +.BR MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ " (since Linux 5.10)" +Register the process's intent to use +.BR MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ . +.TP +.BR MEMBARRIER_CMD_SHARED " (since Linux 4.3)" +This is an alias for +.B MEMBARRIER_CMD_GLOBAL +that exists for header backward compatibility. +.PP +The +.I flags +argument must be specified as 0 unless the command is +.BR MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ , +in which case +.I flags +can be either 0 or +.BR MEMBARRIER_CMD_FLAG_CPU . +.PP +The +.I cpu_id +argument is ignored unless +.I flags +is +.BR MEMBARRIER_CMD_FLAG_CPU , +in which case it must specify the CPU targeted by this membarrier +command. +.PP +All memory accesses performed in program order from each targeted thread +are guaranteed to be ordered with respect to +.BR membarrier (). +.PP +If we use the semantic +.I barrier() +to represent a compiler barrier forcing memory +accesses to be performed in program order across the barrier, and +.I smp_mb() +to represent explicit memory barriers forcing full memory +ordering across the barrier, we have the following ordering table for +each pairing of +.IR barrier() , +.BR membarrier (), +and +.IR smp_mb() . +The pair ordering is detailed as (O: ordered, X: not ordered): +.PP +.RS +.TS +l c c c. +\& barrier() smp_mb() membarrier() +barrier() X X O +smp_mb() X O O +membarrier() O O O +.TE +.RE +.SH RETURN VALUE +On success, the +.B MEMBARRIER_CMD_QUERY +operation returns a bit mask of supported commands, and the +.BR MEMBARRIER_CMD_GLOBAL , +.BR MEMBARRIER_CMD_GLOBAL_EXPEDITED , +.BR MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED , +.BR MEMBARRIER_CMD_PRIVATE_EXPEDITED , +.BR MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED , +.BR MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE , +and +.B MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE +operations return zero. +On error, \-1 is returned, +and +.I errno +is set to indicate the error. +.PP +For a given command, with +.I 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 +.I flags +set to 0, error handling is required only for the first call to +.BR membarrier (). +.SH ERRORS +.TP +.B EINVAL +.I cmd +is invalid, or +.I flags +is nonzero, or the +.B MEMBARRIER_CMD_GLOBAL +command is disabled because the +.I nohz_full +CPU parameter has been set, or the +.B MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE +and +.B MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE +commands are not implemented by the architecture. +.TP +.B ENOSYS +The +.BR membarrier () +system call is not implemented by this kernel. +.TP +.B EPERM +The current process was not registered prior to using private expedited +commands. +.SH STANDARDS +Linux. +.SH HISTORY +Linux 4.3. +.PP +Before Linux 5.10, the prototype was: +.PP +.in +4n +.EX +.BI "int membarrier(int " cmd ", int " flags ); +.EE +.in +.SH NOTES +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. +.PP +Program order is the order in which instructions are ordered in the +program assembly code. +.PP +Examples where +.BR membarrier () +can be useful include implementations +of Read-Copy-Update libraries and garbage collectors. +.SH EXAMPLES +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 +.BR membarrier (): +.PP +.in +4n +.\" SRC BEGIN (membarrier.c) +.EX +#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(void) +{ + 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); +} +.EE +.\" SRC END +.in +.PP +The code above transformed to use +.BR membarrier () +becomes: +.PP +.in +4n +.EX +#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, unsigned int flags, int cpu_id) +{ + return syscall(__NR_membarrier, cmd, flags, cpu_id); +} +\& +static int +init_membarrier(void) +{ + int ret; +\& + /* Check that membarrier() is supported. */ +\& + ret = membarrier(MEMBARRIER_CMD_QUERY, 0, 0); + if (ret < 0) { + perror("membarrier"); + return \-1; + } +\& + if (!(ret & MEMBARRIER_CMD_GLOBAL)) { + fprintf(stderr, + "membarrier does not support MEMBARRIER_CMD_GLOBAL\en"); + 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_GLOBAL, 0, 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); +} +.EE +.in +.\" .SH SEE ALSO +.\" FIXME See if the following syscalls make it into Linux 4.15 or later +.\" .BR cpu_opv (2), +.\" .BR rseq (2) |