On Sun, Jul 29, 2018 at 8:51 AM, Alexei Starovoitov <
alexei.starovoitov@gmail.com> wrote:

On Thu, Jul 26, 2018 at 7:51 PM, Daniel Colascione [off-list ref]
wrote:

quoted

BPF_SYNCHRONIZE_MAPS waits for the release of any references to a BPF
map made by a BPF program that is running at the time the
BPF_SYNCHRONIZE_MAPS command is issued. The purpose of this command is
to provide a means for userspace to replace a BPF map with another,
newer version, then ensure that no component is still using the "old"
map before manipulating the "old" map in some way.

Signed-off-by: Daniel Colascione <redacted>
---
 include/uapi/linux/bpf.h |  9 +++++++++
 kernel/bpf/syscall.c     | 13 +++++++++++++
 2 files changed, 22 insertions(+)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index b7db3261c62d..5b27e9117d3e 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h

@@ -75,6 +75,14 @@ struct bpf_lpm_trie_key {
        __u8    data[0];        /* Arbitrary size */
 };

+/* BPF_SYNCHRONIZE_MAPS waits for the release of any references to a
+ * BPF map made by a BPF program that is running at the time the
+ * BPF_SYNCHRONIZE_MAPS command is issued. The purpose of this command

that doesn't sound right to me.
such command won't wait for the release of the references.
in case of map-in-map the program does not hold
the references to inner map (only to outer map).

Well, today that's the guarantee it provides. :-) I figured it'd be simpler
to explain the guarantee this way.

How about "waits for the release of any reference to a map obtained from
another map"?

quoted

+ * is to provide a means for userspace to replace a BPF map with
+ * another, newer version, then ensure that no component is still
+ * using the "old" map before manipulating the "old" map in some way.
+ */

that's correct, but the whole paragraph still reads too
'generic' which will lead to confusion,
whereas the use case is map-in-map only.
I think bpf_sync_inner_map or
bpf_sync_map_in_map would be better
choices for the name.

I worry that a name like that would be too specific.