Hello Mathieu,
On 12 February 2018 at 20:55, Mathieu Desnoyers
<mathieu.desnoyers@xxxxxxxxxxxx> wrote:
> Document the following membarrier commands introduced in 4.16:
> - MEMBARRIER_CMD_GLOBAL_EXPEDITED (the old enum label
> MEMBARRIER_CMD_SHARED is now an alias to preserve header backward
> compatibility),
> - MEMBARRIER_CMD_GLOBAL_EXPEDITED,
> - MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED,
> - MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE,
> - MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE.
>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> CC: Michael Kerrisk <mtk.manpages@xxxxxxxxx>
> CC: Ingo Molnar <mingo@xxxxxxxxxx>
> CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> ---
> man2/membarrier.2 | 73 ++++++++++++++++++++++++++++++++++++++++++++++---------
> 1 file changed, 62 insertions(+), 11 deletions(-)
>
> diff --git a/man2/membarrier.2 b/man2/membarrier.2
> index c47bc875a..e878301ca 100644
> --- a/man2/membarrier.2
> +++ b/man2/membarrier.2
> @@ -80,7 +80,7 @@ This command is always supported (on kernels where
> .BR membarrier ()
> is provided).
> .TP
> -.B MEMBARRIER_CMD_SHARED
> +.B 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
> @@ -88,7 +88,30 @@ order between entry to and return from the
> system call.
> All threads on the system are targeted by this command.
> .TP
> -.BR MEMBARRIER_CMD_PRIVATE_EXPEDITED " (since Linux 4.14)"
> +.B MEMBARRIER_CMD_GLOBAL_EXPEDITED " (since Linux 4.16)"
> +Execute a memory barrier on all running threads of all processes which
> +previously registered with
> +.BR MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED .
> +Upon return from system call, the caller thread is ensured 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 only covers threads from processes which registered with
> +.BR MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED .
> +Given that registration is about the intent to receive the barriers, it
> +is valid to invoke
> +.BR MEMBARRIER_CMD_GLOBAL_EXPEDITED
> +from a non-registered process.
> +.IP
> +The "expedited" commands complete faster than the non-expedited ones;
> +they never block, but have the downside of causing extra overhead.
> +.TP
> +.B MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED " (since Linux 4.16)"
> +Register the process intent to receive
> +.BR MEMBARRIER_CMD_GLOBAL_EXPEDITED
> +memory barriers.
> +.TP
> +.B 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
> @@ -103,9 +126,29 @@ 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)"
> +.B MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED " (since Linux 4.14)"
> Register the process's intent to use
> -.BR MEMBARRIER_CMD_PRIVATE_EXPEDITED .
> +.B MEMBARRIER_CMD_PRIVATE_EXPEDITED .
> +.TP
> +.B MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE " (since Linux 4.16)"
> +In addition to provide memory ordering guarantees described in
> +.BR MEMBARRIER_CMD_PRIVATE_EXPEDITED ,
> +ensure the caller thread, upon return from system call, that all its
> +running threads siblings have executed a core serializing instruction.
> +This only covers threads from the same process as the caller thread.
> +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 sync
> +core command prior to using it.
> +.TP
> +.B MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE " (since Linux 4.16)"
> +Register the process intent to use
> +.BR MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE .
> +.TP
> +.B MEMBARRIER_CMD_SHARED
> + Alias to
> +.BR MEMBARRIER_CMD_GLOBAL .
> +Provided for header backward compatibility.
> .PP
> The
> .I flags
> @@ -137,10 +180,14 @@ The pair ordering is detailed as (O: ordered, X: not ordered):
> On success, the
> .B MEMBARRIER_CMD_QUERY
> operation returns a bit mask of supported commands, and the
> -.B MEMBARRIER_CMD_SHARED ,
> +.B MEMBARRIER_CMD_GLOBAL ,
> +.B MEMBARRIER_CMD_GLOBAL_EXPEDITED ,
> +.B MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED ,
> .B MEMBARRIER_CMD_PRIVATE_EXPEDITED ,
> -and
> .B MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED ,
> +.B MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE ,
> +and
> +.B MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE
> operations return zero.
> On error, \-1 is returned,
> and
> @@ -163,10 +210,14 @@ set to 0, error handling is required only for the first call to
> is invalid, or
> .I flags
> is nonzero, or the
> -.BR MEMBARRIER_CMD_SHARED
> +.BR MEMBARRIER_CMD_GLOBAL
> command is disabled because the
> .I nohz_full
> -CPU parameter has been set.
> +CPU parameter has been set, or the
> +.BR MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE
> +and
> +.BR MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE
> +commands are not implemented by the architecture.
> .TP
> .B ENOSYS
> The
> @@ -294,9 +345,9 @@ init_membarrier(void)
> return \-1;
> }
>
> - if (!(ret & MEMBARRIER_CMD_SHARED)) {
> + if (!(ret & MEMBARRIER_CMD_GLOBAL)) {
> fprintf(stderr,
> - "membarrier does not support MEMBARRIER_CMD_SHARED\\n");
> + "membarrier does not support MEMBARRIER_CMD_GLOBAL\\n");
> return \-1;
> }
>
> @@ -315,7 +366,7 @@ static void
> slow_path(int *read_a)
> {
> b = 1;
> - membarrier(MEMBARRIER_CMD_SHARED, 0);
> + membarrier(MEMBARRIER_CMD_GLOBAL, 0);
> *read_a = a;
> }
I have applied the above patch, and done quite a bit of tweaking, and
pushed the results to the git repo.
I would be grateful if you would read the entire manual page as it
currently stands, to see if anything needs improving. I isolated some
of the more significant changes into a simple patch, shown below, and
especially I'd like your confirmation that all of those changes are
okay.
Cheers,
Michael
diff --git a/man2/membarrier.2 b/man2/membarrier.2
index b3a94f95f..81d573dd5 100644
--- a/man2/membarrier.2
+++ b/man2/membarrier.2
@@ -92,16 +92,18 @@ All threads on the system are targeted by this command.
Execute a memory barrier on all running threads of all processes that
previously registered with
.BR MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED .
-Upon return from the system call, the calling thread is ensured that all
+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 covers only threads from processes which registered with
+This guarantee is provided only for the threads of processes that
+previously registered with
.BR MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED .
Given that registration is about the intent to receive the barriers, it
is valid to invoke
.BR MEMBARRIER_CMD_GLOBAL_EXPEDITED
-from a non-registered process.
+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.
@@ -113,17 +115,18 @@ 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 current thread.
-Upon return from system call, the calling
-thread is assured that all its running threads siblings have passed
+process as the calling thread.
+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 covers only threads from the same process as the calling thread.
+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.
-A process needs to register its intent to use the private
+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)"
@@ -133,12 +136,13 @@ Register the process's intent to use
.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 ,
-ensure the calling thread, upon return from system call, that all its
-running threads siblings have executed a core serializing instruction.
-This only covers threads from the same process as the calling thread.
+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.
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 sync
+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)"
--
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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