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Subject: Re: [RFC PATCH 3/3] Update membarrier manpage for 4.14, 4.15
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
References: <20171116151711.19001-1-mathieu.desnoyers@efficios.com>
 <20171116151711.19001-4-mathieu.desnoyers@efficios.com>
From:   "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
Message-ID: <038c0532-f45c-7beb-5b47-e34816f5b082@gmail.com>
Date:   Sun, 19 Nov 2017 12:12:10 +0100
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Hi Mathieu,

On 11/16/2017 04:17 PM, Mathieu Desnoyers wrote:
> Add documentation for those new membarrier commands:
> 
> New in 4.14:
>         MEMBARRIER_CMD_PRIVATE_EXPEDITED
>         MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED
> 
> Adapt the MEMBARRIER_CMD_SHARED return value documentation to reflect
> that it now returns -EINVAL when issued on a system configured for
> nohz_full.
> 
> New in 4.15:
>         MEMBARRIER_CMD_SHARED_EXPEDITED
>         MEMBARRIER_CMD_REGISTER_SHARED_EXPEDITED

Combining details into one patch in this way makes things more difficult
for me. I would prefer these details to be split into separate pieces
for the 4.14 and 4.15 changes, in part because of the way the man-pages
release cycle currently works (roughly synchronized with kernel releases)
and also because the 4.15 details might change.

I've applied the 4.14 details from this patch to the man page; if you had a 
chance to check whether I did that job correctly, that would be great.
See the patch below.

Once the pieces get merged for 4.15, could you ping me/resend a patch for
the 4.15 changes please.

Cheers,

Michael


diff --git a/man2/membarrier.2 b/man2/membarrier.2
index bbf611e10..6390f9693 100644
--- a/man2/membarrier.2
+++ b/man2/membarrier.2
@@ -1,4 +1,4 @@
-.\" Copyright 2015 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
+.\" Copyright 2015-2017 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
 .\"
 .\" %%%LICENSE_START(VERBATIM)
 .\" Permission is granted to make and distribute verbatim copies of this
@@ -22,7 +22,7 @@
 .\" the source, must acknowledge the copyright and authors of this work.
 .\" %%%LICENSE_END
 .\"
-.TH MEMBARRIER 2 2017-09-15 "Linux" "Linux Programmer's Manual"
+.TH MEMBARRIER 2 2017-11-15 "Linux" "Linux Programmer's Manual"
 .SH NAME
 membarrier \- issue memory barriers on a set of threads
 .SH SYNOPSIS
@@ -87,6 +87,25 @@ 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_PRIVATE_EXPEDITED " (since Linux 4.14)"
+Execute a memory barrier on each running thread belonging to the same
+process as the current thread.
+Upon return from system call, the calling
+thread is assured that all its running threads 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 covers only threads from 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.
+A process needs to 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 .
 .PP
 The
 .I flags
@@ -117,9 +136,12 @@ The pair ordering is detailed as (O: ordered, X: not ordered):
 .SH RETURN VALUE
 On success, the
 .B MEMBARRIER_CMD_QUERY
-operation returns a bit mask of supported commands and the
-.B MEMBARRIER_CMD_SHARED
-operation returns zero.
+operation returns a bit mask of supported commands, and the
+.B MEMBARRIER_CMD_SHARED ,
+.B MEMBARRIER_CMD_PRIVATE_EXPEDITED ,
+and
+.B MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED ,
+operations return zero.
 On error, \-1 is returned,
 and
 .I errno
@@ -138,22 +160,22 @@ set to 0, error handling is required only for the first call to
 .TP
 .B EINVAL
 .I cmd
-is invalid or
+is invalid, or
 .I flags
-is non-zero.
+is non-zero, or the
+.BR MEMBARRIER_CMD_SHARED
+command is disabled because the
+.I nohz_full
+CPU parameter has been set.
 .TP
 .B ENOSYS
 The
 .BR membarrier ()
 system call is not implemented by this kernel.
 .TP
-.BR ENOSYS " (since Linux 4.11)"
-.\" 907565337ebf998a68cb5c5b2174ce5e5da065eb
-The
-.BR membarrier ()
-system call is disabled because the
-.I nohz_full
-CPU parameter has been set.
+.B EPERM
+The current process was not registered prior to using private expedited
+commands.
 .SH VERSIONS
 The
 .BR membarrier ()
@@ -162,6 +184,10 @@ system call was added in Linux 4.3.
 .SH CONFORMING TO
 .BR membarrier ()
 is Linux-specific.
+.\" .SH SEE ALSO
+.\" FIXME See if the following syscalls make it into Linux 4.15 or later
+.\" .BR cpu_opv (2),
+.\" .BR rseq (2)
 .SH NOTES
 A memory barrier instruction is part of the instruction set of
 architectures with weakly-ordered memory models.

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/

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