[PATCH] membarrier.2: Update membarrier manpage for 5.10

[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).
---
 man2/membarrier.2 | 41 +++++++++++++++++++++++++++++++++++++++--
 1 file changed, 39 insertions(+), 2 deletions(-)

diff --git a/man2/membarrier.2 b/man2/membarrier.2
index 3064b2d2e..c95e97cb6 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
-- 
2.29.1.341.ge80a0c044ae-goog