[PATCH v3] membarrier.2: Update membarrier manual page for 5.10

Alejandro Colomar
2020-11-04T16:30:29Z

From: Peter Oskolkov <posk@google.com>

Linux kernel commit 2a36ab717e8fe678d98f81c14a0b124712719840
(part of 5.10 release) changed sys_membarrier prototype/parameters
and added two new commands. This manpages patch reflects these
changes, by mostly copying comments from the kernel patch
into the manpage (I was also the author of the kernel change).

Signed-off-by: Peter Oskolkov <posk@google.com>
Co-developed-by: Alejandro Colomar <colomar.6.4.3@gmail.com>
Signed-off-by: Alejandro Colomar <colomar.6.4.3@gmail.com>
---

On 2020-11-04 17:01, Peter Oskolkov wrote:
> On Wed, Nov 4, 2020 at 1:39 AM Alejandro Colomar
> <colomar.6.4.3@gmail.com> wrote:
>>
>> Maybe I'd also add a paragraph in VERSIONS, saying there was an old
>> version with a different prototype (and maybe show it there).
>>
>> What do you think about it?
>
> Hi Alex,
>
> I don't mind your re-doing my patch with the typo fix, updated
> examples, and the addition to versions. Please
> let me know if you would like to do that. If not, I'll send a new
> patch with these changes.
>
> Thanks,
> Peter

Hi Peter,

I added those changes to the patch,
and added a signed-off-by on your behalf.
I think it's ready right now.

Cheers,

Alex

man2/membarrier.2 | 60 +++++++++++++++++++++++++++++++++++++++++------
1 file changed, 53 insertions(+), 7 deletions(-)

diff --git a/man2/membarrier.2 b/man2/membarrier.2
index 3064b2d2e..775a399b0 100644
--- a/man2/membarrier.2
+++ b/man2/membarrier.2
@@ -30,7 +30,7 @@ membarrier \- issue memory barriers on a set of threads
.PP
.B #include <linux/membarrier.h>
.PP
-.BI "int membarrier(int " cmd ", int " flags ");"
+.BI "int membarrier(int " cmd ", unsigned int " flags ", int " cpu_id );
.fi
.PP
.IR Note :
@@ -165,6 +165,29 @@ core command prior to using it.
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
+.I 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
.BR MEMBARRIER_CMD_GLOBAL
@@ -172,7 +195,21 @@ that exists for header backward compatibility.
.PP
The
.I flags
-argument is currently unused and must be specified as 0.
+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
@@ -251,7 +288,16 @@ commands.
The
.BR membarrier ()
system call was added in Linux 4.3.
-.\"
+.PP
+Before Linux 5.10, the prototype for
+.BR membarrier ()
+was:
+.PP
+.in +4n
+.EX
+.BI "int membarrier(int " cmd ", int " flags );
+.EE
+.in
.SH CONFORMING TO
.BR membarrier ()
is Linux-specific.
@@ -350,9 +396,9 @@ becomes:
static volatile int a, b;

static int
-membarrier(int cmd, int flags)
+membarrier(int cmd, unsigned int flags, int cpu_id)
{
- return syscall(__NR_membarrier, cmd, flags);
+ return syscall(__NR_membarrier, cmd, flags, cpu_id);
}

static int
@@ -362,7 +408,7 @@ init_membarrier(void)

/* Check that membarrier() is supported. */

- ret = membarrier(MEMBARRIER_CMD_QUERY, 0);
+ ret = membarrier(MEMBARRIER_CMD_QUERY, 0, 0);
if (ret < 0) {
perror("membarrier");
return \-1;
@@ -389,7 +435,7 @@ static void
slow_path(int *read_a)
{
b = 1;
- membarrier(MEMBARRIER_CMD_GLOBAL, 0);
+ membarrier(MEMBARRIER_CMD_GLOBAL, 0, 0);
*read_a = a;
}

--
2.28.0