mirror of https://github.com/mkerrisk/man-pages
475 lines
13 KiB
Groff
475 lines
13 KiB
Groff
.\" Copyright 2015-2017 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.TH MEMBARRIER 2 2021-08-27 "Linux" "Linux Programmer's Manual"
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.SH NAME
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membarrier \- issue memory barriers on a set of threads
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.SH SYNOPSIS
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.nf
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.PP
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.BR "#include <linux/membarrier.h>" \
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" /* Definition of " MEMBARRIER_* " constants */"
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.BR "#include <sys/syscall.h>" " /* Definition of " SYS_* " constants */"
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.B #include <unistd.h>
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.PP
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.BI "int syscall(SYS_membarrier, int " cmd ", unsigned int " flags \
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", int " cpu_id );
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.fi
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.PP
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.IR Note :
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glibc provides no wrapper for
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.BR membarrier (),
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necessitating the use of
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.BR syscall (2).
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.SH DESCRIPTION
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The
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.BR membarrier ()
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system call helps reducing the overhead of the memory barrier
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instructions required to order memory accesses on multi-core systems.
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However, this system call is heavier than a memory barrier, so using it
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effectively is
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.I not
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as simple as replacing memory barriers with this
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system call, but requires understanding of the details below.
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.PP
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Use of memory barriers needs to be done taking into account that a
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memory barrier always needs to be either matched with its memory barrier
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counterparts, or that the architecture's memory model doesn't require the
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matching barriers.
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.PP
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There are cases where one side of the matching barriers (which we will
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refer to as "fast side") is executed much more often than the other
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(which we will refer to as "slow side").
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This is a prime target for the use of
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.BR membarrier ().
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The key idea is to replace, for these matching
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barriers, the fast-side memory barriers by simple compiler barriers,
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for example:
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.PP
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.in +4n
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.EX
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asm volatile ("" : : : "memory")
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.EE
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.in
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.PP
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and replace the slow-side memory barriers by calls to
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.BR membarrier ().
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.PP
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This will add overhead to the slow side, and remove overhead from the
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fast side, thus resulting in an overall performance increase as long as
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the slow side is infrequent enough that the overhead of the
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.BR membarrier ()
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calls does not outweigh the performance gain on the fast side.
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.PP
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The
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.I cmd
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argument is one of the following:
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.TP
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.BR MEMBARRIER_CMD_QUERY " (since Linux 4.3)"
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Query the set of supported commands.
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The return value of the call is a bit mask of supported
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commands.
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.BR MEMBARRIER_CMD_QUERY ,
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which has the value 0,
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is not itself included in this bit mask.
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This command is always supported (on kernels where
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.BR membarrier ()
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is provided).
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.TP
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.BR MEMBARRIER_CMD_GLOBAL " (since Linux 4.16)"
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Ensure that all threads from all processes on the system pass through a
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state where all memory accesses to user-space addresses match program
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order between entry to and return from the
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.BR membarrier ()
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system call.
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All threads on the system are targeted by this command.
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.TP
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.BR MEMBARRIER_CMD_GLOBAL_EXPEDITED " (since Linux 4.16)"
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Execute a memory barrier on all running threads of all processes that
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previously registered with
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.BR MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED .
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.IP
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Upon return from the system call, the calling thread has a guarantee that all
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running threads have passed through a state where all memory accesses to
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user-space addresses match program order between entry to and return
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from the system call (non-running threads are de facto in such a state).
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This guarantee is provided only for the threads of processes that
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previously registered with
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.BR MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED .
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.IP
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Given that registration is about the intent to receive the barriers, it
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is valid to invoke
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.BR MEMBARRIER_CMD_GLOBAL_EXPEDITED
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from a process that has not employed
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.BR MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED .
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.IP
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The "expedited" commands complete faster than the non-expedited ones;
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they never block, but have the downside of causing extra overhead.
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.TP
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.BR MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED " (since Linux 4.16)"
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Register the process's intent to receive
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.BR MEMBARRIER_CMD_GLOBAL_EXPEDITED
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memory barriers.
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.TP
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.BR MEMBARRIER_CMD_PRIVATE_EXPEDITED " (since Linux 4.14)"
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Execute a memory barrier on each running thread belonging to the same
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process as the calling thread.
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.IP
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Upon return from the system call, the calling
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thread has a guarantee that all its running thread siblings have passed
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through a state where all memory accesses to user-space addresses match
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program order between entry to and return from the system call
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(non-running threads are de facto in such a state).
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This guarantee is provided only for threads in
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the same process as the calling thread.
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.IP
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The "expedited" commands complete faster than the non-expedited ones;
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they never block, but have the downside of causing extra overhead.
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.IP
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A process must register its intent to use the private
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expedited command prior to using it.
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.TP
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.BR MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED " (since Linux 4.14)"
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Register the process's intent to use
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.BR MEMBARRIER_CMD_PRIVATE_EXPEDITED .
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.TP
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.BR MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE " (since Linux 4.16)"
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In addition to providing the memory ordering guarantees described in
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.BR MEMBARRIER_CMD_PRIVATE_EXPEDITED ,
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upon return from system call the calling thread has a guarantee that all its
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running thread siblings have executed a core serializing instruction.
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This guarantee is provided only for threads in
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the same process as the calling thread.
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.IP
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The "expedited" commands complete faster than the non-expedited ones,
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they never block, but have the downside of causing extra overhead.
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.IP
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A process must register its intent to use the private expedited sync
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core command prior to using it.
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.TP
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.BR MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE " (since Linux 4.16)"
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Register the process's intent to use
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.BR MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE .
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.TP
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.BR MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ " (since Linux 5.10)"
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Ensure the caller thread, upon return from system call, that all its
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running thread siblings have any currently running rseq critical sections
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restarted if
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.I flags
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parameter is 0; if
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.I flags
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parameter is
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.BR MEMBARRIER_CMD_FLAG_CPU ,
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then this operation is performed only on CPU indicated by
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.IR cpu_id .
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This guarantee is provided only for threads in
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the same process as the calling thread.
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.IP
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RSEQ membarrier is only available in the "private expedited" form.
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.IP
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A process must register its intent to use the private expedited rseq
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command prior to using it.
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.TP
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.BR MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ " (since Linux 5.10)"
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Register the process's intent to use
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.BR MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ .
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.TP
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.BR MEMBARRIER_CMD_SHARED " (since Linux 4.3)"
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This is an alias for
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.BR MEMBARRIER_CMD_GLOBAL
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that exists for header backward compatibility.
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.PP
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The
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.I flags
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argument must be specified as 0 unless the command is
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.BR MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ ,
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in which case
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.I flags
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can be either 0 or
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.BR MEMBARRIER_CMD_FLAG_CPU .
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.PP
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The
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.I cpu_id
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argument is ignored unless
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.I flags
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is
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.BR MEMBARRIER_CMD_FLAG_CPU ,
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in which case it must specify the CPU targeted by this membarrier
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command.
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.PP
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All memory accesses performed in program order from each targeted thread
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are guaranteed to be ordered with respect to
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.BR membarrier ().
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.PP
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If we use the semantic
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.I barrier()
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to represent a compiler barrier forcing memory
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accesses to be performed in program order across the barrier, and
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.I smp_mb()
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to represent explicit memory barriers forcing full memory
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ordering across the barrier, we have the following ordering table for
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each pairing of
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.IR barrier() ,
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.BR membarrier (),
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and
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.IR smp_mb() .
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The pair ordering is detailed as (O: ordered, X: not ordered):
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.PP
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barrier() smp_mb() membarrier()
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barrier() X X O
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smp_mb() X O O
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membarrier() O O O
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.SH RETURN VALUE
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On success, the
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.B MEMBARRIER_CMD_QUERY
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operation returns a bit mask of supported commands, and the
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.BR MEMBARRIER_CMD_GLOBAL ,
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.BR MEMBARRIER_CMD_GLOBAL_EXPEDITED ,
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.BR MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED ,
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.BR MEMBARRIER_CMD_PRIVATE_EXPEDITED ,
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.BR MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED ,
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.BR MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE ,
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and
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.B MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE
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operations return zero.
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On error, \-1 is returned,
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and
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.I errno
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is set to indicate the error.
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.PP
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For a given command, with
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.I flags
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set to 0, this system call is
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guaranteed to always return the same value until reboot.
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Further calls with the same arguments will lead to the same result.
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Therefore, with
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.I flags
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set to 0, error handling is required only for the first call to
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.BR membarrier ().
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.SH ERRORS
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.TP
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.B EINVAL
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.I cmd
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is invalid, or
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.I flags
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is nonzero, or the
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.BR MEMBARRIER_CMD_GLOBAL
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command is disabled because the
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.I nohz_full
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CPU parameter has been set, or the
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.BR MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE
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and
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.BR MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE
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commands are not implemented by the architecture.
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.TP
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.B ENOSYS
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The
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.BR membarrier ()
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system call is not implemented by this kernel.
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.TP
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.B EPERM
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The current process was not registered prior to using private expedited
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commands.
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.SH VERSIONS
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The
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.BR membarrier ()
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system call was added in Linux 4.3.
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.PP
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Before Linux 5.10, the prototype for
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.BR membarrier ()
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was:
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.PP
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.in +4n
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.EX
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.BI "int membarrier(int " cmd ", int " flags );
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.EE
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.in
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.SH CONFORMING TO
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.BR membarrier ()
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is Linux-specific.
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.\" .SH SEE ALSO
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.\" FIXME See if the following syscalls make it into Linux 4.15 or later
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.\" .BR cpu_opv (2),
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.\" .BR rseq (2)
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.SH NOTES
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A memory barrier instruction is part of the instruction set of
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architectures with weakly ordered memory models.
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It orders memory
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accesses prior to the barrier and after the barrier with respect to
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matching barriers on other cores.
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For instance, a load fence can order
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loads prior to and following that fence with respect to stores ordered
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by store fences.
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.PP
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Program order is the order in which instructions are ordered in the
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program assembly code.
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.PP
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Examples where
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.BR membarrier ()
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can be useful include implementations
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of Read-Copy-Update libraries and garbage collectors.
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.SH EXAMPLES
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Assuming a multithreaded application where "fast_path()" is executed
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very frequently, and where "slow_path()" is executed infrequently, the
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following code (x86) can be transformed using
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.BR membarrier ():
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.PP
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.in +4n
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.EX
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#include <stdlib.h>
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static volatile int a, b;
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static void
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fast_path(int *read_b)
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{
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a = 1;
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asm volatile ("mfence" : : : "memory");
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*read_b = b;
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}
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static void
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slow_path(int *read_a)
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{
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b = 1;
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asm volatile ("mfence" : : : "memory");
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*read_a = a;
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}
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int
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main(int argc, char *argv[])
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{
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int read_a, read_b;
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/*
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* Real applications would call fast_path() and slow_path()
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* from different threads. Call those from main() to keep
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* this example short.
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*/
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slow_path(&read_a);
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fast_path(&read_b);
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/*
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* read_b == 0 implies read_a == 1 and
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* read_a == 0 implies read_b == 1.
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*/
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if (read_b == 0 && read_a == 0)
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abort();
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exit(EXIT_SUCCESS);
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}
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.EE
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.in
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.PP
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The code above transformed to use
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.BR membarrier ()
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becomes:
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.PP
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.in +4n
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.EX
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#define _GNU_SOURCE
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#include <stdlib.h>
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#include <stdio.h>
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#include <unistd.h>
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#include <sys/syscall.h>
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#include <linux/membarrier.h>
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static volatile int a, b;
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static int
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membarrier(int cmd, unsigned int flags, int cpu_id)
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{
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return syscall(__NR_membarrier, cmd, flags, cpu_id);
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}
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static int
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init_membarrier(void)
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{
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int ret;
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/* Check that membarrier() is supported. */
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ret = membarrier(MEMBARRIER_CMD_QUERY, 0, 0);
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if (ret < 0) {
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perror("membarrier");
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return \-1;
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}
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if (!(ret & MEMBARRIER_CMD_GLOBAL)) {
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fprintf(stderr,
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"membarrier does not support MEMBARRIER_CMD_GLOBAL\en");
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return \-1;
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}
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return 0;
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}
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static void
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fast_path(int *read_b)
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{
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a = 1;
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asm volatile ("" : : : "memory");
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*read_b = b;
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}
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static void
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slow_path(int *read_a)
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{
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b = 1;
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membarrier(MEMBARRIER_CMD_GLOBAL, 0, 0);
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*read_a = a;
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}
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int
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main(int argc, char *argv[])
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{
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int read_a, read_b;
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if (init_membarrier())
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exit(EXIT_FAILURE);
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/*
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* Real applications would call fast_path() and slow_path()
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* from different threads. Call those from main() to keep
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* this example short.
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*/
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slow_path(&read_a);
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fast_path(&read_b);
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/*
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* read_b == 0 implies read_a == 1 and
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* read_a == 0 implies read_b == 1.
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*/
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if (read_b == 0 && read_a == 0)
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abort();
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exit(EXIT_SUCCESS);
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}
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.EE
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.in
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