Re: [PATCH v4] membarrier.2: Update membarrier manual page for 5.10

Michael Kerrisk (man-pages)
2020-11-05T12:04:36Z

Hello Peter and Alex,

On 11/5/20 12:32 PM, Alejandro Colomar wrote:
> 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).

Thanks for this patch! And adding that last detail in the commit
message saves me some time, so thanks for noting you are the
author of the code also.

Alex, thanks for the additional work on the patch!

Patch applied.

Cheers,

Michael

>
> 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>
> ---
>
> v4 fixes a typo.
>
>
> 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
> +.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
> .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;
> }
>
>

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