On Wed, Jul 29, 2020 at 07:48 PM CEST, Andrii Nakryiko wrote:

On Wed, Jul 29, 2020 at 7:29 AM Jakub Sitnicki [off-list ref] wrote:

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On Wed, Jul 29, 2020 at 06:09 AM CEST, Andrii Nakryiko wrote:

[...]

quoted

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+/*
+ * Trigger synchronize_cpu() in kernel.

Nit: synchronize_*r*cu().

welp, yeah

quoted

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+ *
+ * ARRAY_OF_MAPS/HASH_OF_MAPS lookup/update operations trigger
+ * synchronize_rcu(), if looking up/updating non-NULL element. Use this fact
+ * to trigger synchronize_cpu(): create map-in-map, create a trivial ARRAY
+ * map, update map-in-map with ARRAY inner map. Then cleanup. At the end, at
+ * least one synchronize_rcu() would be called.
+ */

That's a cool trick. I'm a bit confused by "looking up/updating non-NULL
element". It looks like you're updating an element that is NULL/unset in
the code below. What am I missing?

I was basically trying to say that it has to be a successful lookup or
update. For lookup that means looking up non-NULL (existing) entry.
For update -- setting valid inner map FD.

Not sure fixing this and typo above is worth it to post v5.

I just wanted to understand that the helper is working as intended. It
seems handy. I agree that it's not worth respinning the patches just for
this.

quoted

quoted

+static int kern_sync_rcu(void)
+{
+     int inner_map_fd, outer_map_fd, err, zero = 0;
+
+     inner_map_fd = bpf_create_map(BPF_MAP_TYPE_ARRAY, 4, 4, 1, 0);
+     if (CHECK(inner_map_fd < 0, "inner_map_create", "failed %d\n", -errno))
+             return -1;
+
+     outer_map_fd = bpf_create_map_in_map(BPF_MAP_TYPE_ARRAY_OF_MAPS, NULL,
+                                          sizeof(int), inner_map_fd, 1, 0);
+     if (CHECK(outer_map_fd < 0, "outer_map_create", "failed %d\n", -errno)) {
+             close(inner_map_fd);
+             return -1;
+     }
+
+     err = bpf_map_update_elem(outer_map_fd, &zero, &inner_map_fd, 0);
+     if (err)
+             err = -errno;
+     CHECK(err, "outer_map_update", "failed %d\n", err);
+     close(inner_map_fd);
+     close(outer_map_fd);
+     return err;
+}
+

[...]

trimming's good ;)

You caught me. Just being lazy. No excuses :-)