selftests/bpf: Add test case for atomic update of fd htab

Add a test case to verify the atomic update of existing elements in the
htab of maps. The test proceeds in three steps:

1) fill the outer map with keys in the range [0, 8]
For each inner array map, the value of its first element is set as the
key used to lookup the inner map.

2) create 16 threads to lookup these keys concurrently
Each lookup thread first lookups the inner map, then it checks whether
the first value of the inner array map is the same as the key used to
lookup the inner map.

3) create 8 threads to overwrite these keys concurrently
Each update thread first creates an inner array, it sets the first value
of the array to the key used to update the outer map, then it uses the
key and the inner map to update the outer map.

Without atomic update support, the lookup operation may return -ENOENT
during the lookup of outer map, or return -EINVAL during the comparison
of the first value in the inner map and the key used for inner map, and
the test will fail. After the atomic update change, both the lookup and
the comparison will succeed.

Given that the update of outer map is slow, the test case sets the loop
number for each thread as 5 to reduce the total running time. However,
the loop number could also be adjusted through FD_HTAB_LOOP_NR
environment variable.

Signed-off-by: Hou Tao <[email protected]>

Author: Hou Tao

tools/testing/selftests/bpf/prog_tests/fd_htab_lookup.c

@@ -0,0 +1,192 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (C) 2025. Huawei Technologies Co., Ltd */
+#define _GNU_SOURCE
+#include <stdbool.h>
+#include <test_progs.h>
+#include "fd_htab_lookup.skel.h"
+
+struct htab_op_ctx {
+	int fd;
+	int loop;
+	unsigned int entries;
+	bool stop;
+};
+
+#define ERR_TO_RETVAL(where, err) ((void *)(long)(((where) << 12) | (-err)))
+
+static void *htab_lookup_fn(void *arg)
+{
+	struct htab_op_ctx *ctx = arg;
+	int i = 0;
+
+	while (i++ < ctx->loop && !ctx->stop) {
+		unsigned int j;
+
+		for (j = 0; j < ctx->entries; j++) {
+			unsigned int key = j, zero = 0, value;
+			int inner_fd, err;
+
+			err = bpf_map_lookup_elem(ctx->fd, &key, &value);
+			if (err) {
+				ctx->stop = true;
+				return ERR_TO_RETVAL(1, err);
+			}
+
+			inner_fd = bpf_map_get_fd_by_id(value);
+			if (inner_fd < 0) {
+				/* The old map has been freed */
+				if (inner_fd == -ENOENT)
+					continue;
+				ctx->stop = true;
+				return ERR_TO_RETVAL(2, inner_fd);
+			}
+
+			err = bpf_map_lookup_elem(inner_fd, &zero, &value);
+			if (err) {
+				close(inner_fd);
+				ctx->stop = true;
+				return ERR_TO_RETVAL(3, err);
+			}
+			close(inner_fd);
+
+			if (value != key) {
+				ctx->stop = true;
+				return ERR_TO_RETVAL(4, -EINVAL);
+			}
+		}
+	}
+
+	return NULL;
+}
+
+static void *htab_update_fn(void *arg)
+{
+	struct htab_op_ctx *ctx = arg;
+	int i = 0;
+
+	while (i++ < ctx->loop && !ctx->stop) {
+		unsigned int j;
+
+		for (j = 0; j < ctx->entries; j++) {
+			unsigned int key = j, zero = 0;
+			int inner_fd, err;
+
+			inner_fd = bpf_map_create(BPF_MAP_TYPE_ARRAY, NULL, 4, 4, 1, NULL);
+			if (inner_fd < 0) {
+				ctx->stop = true;
+				return ERR_TO_RETVAL(1, inner_fd);
+			}
+
+			err = bpf_map_update_elem(inner_fd, &zero, &key, 0);
+			if (err) {
+				close(inner_fd);
+				ctx->stop = true;
+				return ERR_TO_RETVAL(2, err);
+			}
+
+			err = bpf_map_update_elem(ctx->fd, &key, &inner_fd, BPF_EXIST);
+			if (err) {
+				close(inner_fd);
+				ctx->stop = true;
+				return ERR_TO_RETVAL(3, err);
+			}
+			close(inner_fd);
+		}
+	}
+
+	return NULL;
+}
+
+static int setup_htab(int fd, unsigned int entries)
+{
+	unsigned int i;
+
+	for (i = 0; i < entries; i++) {
+		unsigned int key = i, zero = 0;
+		int inner_fd, err;
+
+		inner_fd = bpf_map_create(BPF_MAP_TYPE_ARRAY, NULL, 4, 4, 1, NULL);
+		if (!ASSERT_OK_FD(inner_fd, "new array"))
+			return -1;
+
+		err = bpf_map_update_elem(inner_fd, &zero, &key, 0);
+		if (!ASSERT_OK(err, "init array")) {
+			close(inner_fd);
+			return -1;
+		}
+
+		err = bpf_map_update_elem(fd, &key, &inner_fd, 0);
+		if (!ASSERT_OK(err, "init outer")) {
+			close(inner_fd);
+			return -1;
+		}
+		close(inner_fd);
+	}
+
+	return 0;
+}
+
+static int get_int_from_env(const char *name, int dft)
+{
+	const char *value;
+
+	value = getenv(name);
+	if (!value)
+		return dft;
+
+	return atoi(value);
+}
+
+void test_fd_htab_lookup(void)
+{
+	unsigned int i, wr_nr = 8, rd_nr = 16;
+	pthread_t tids[wr_nr + rd_nr];
+	struct fd_htab_lookup *skel;
+	struct htab_op_ctx ctx;
+	int err;
+
+	skel = fd_htab_lookup__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "fd_htab_lookup__open_and_load"))
+		return;
+
+	ctx.fd = bpf_map__fd(skel->maps.outer_map);
+	ctx.loop = get_int_from_env("FD_HTAB_LOOP_NR", 5);
+	ctx.stop = false;
+	ctx.entries = 8;
+
+	err = setup_htab(ctx.fd, ctx.entries);
+	if (err)
+		goto destroy;
+
+	memset(tids, 0, sizeof(tids));
+	for (i = 0; i < wr_nr; i++) {
+		err = pthread_create(&tids[i], NULL, htab_update_fn, &ctx);
+		if (!ASSERT_OK(err, "pthread_create")) {
+			ctx.stop = true;
+			goto reap;
+		}
+	}
+	for (i = 0; i < rd_nr; i++) {
+		err = pthread_create(&tids[i + wr_nr], NULL, htab_lookup_fn, &ctx);
+		if (!ASSERT_OK(err, "pthread_create")) {
+			ctx.stop = true;
+			goto reap;
+		}
+	}
+
+reap:
+	for (i = 0; i < wr_nr + rd_nr; i++) {
+		void *ret = NULL;
+		char desc[32];
+
+		if (!tids[i])
+			continue;
+
+		snprintf(desc, sizeof(desc), "thread %u", i + 1);
+		err = pthread_join(tids[i], &ret);
+		ASSERT_OK(err, desc);
+		ASSERT_EQ(ret, NULL, desc);
+	}
+destroy:
+	fd_htab_lookup__destroy(skel);
+}

tools/testing/selftests/bpf/progs/fd_htab_lookup.c

@@ -0,0 +1,25 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (C) 2025. Huawei Technologies Co., Ltd */
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+
+char _license[] SEC("license") = "GPL";
+
+struct inner_map_type {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__uint(key_size, 4);
+	__uint(value_size, 4);
+	__uint(max_entries, 1);
+} inner_map SEC(".maps");
+
+struct {
+	__uint(type, BPF_MAP_TYPE_HASH_OF_MAPS);
+	__uint(max_entries, 64);
+	__type(key, int);
+	__type(value, int);
+	__array(values, struct inner_map_type);
+} outer_map SEC(".maps") = {
+	.values = {
+		[0] = &inner_map,
+	},
+};