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;
}
--
2.11.0
--
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