selftests/bpf: Test concurrent updates on bpf_task_storage_busy

Under full preemptible kernel, task local storage lookup operations on
the same CPU may update per-cpu bpf_task_storage_busy concurrently. If
the update of bpf_task_storage_busy is not preemption safe, the final
value of bpf_task_storage_busy may become not-zero forever and
bpf_task_storage_trylock() will always fail. So add a test case to
ensure the update of bpf_task_storage_busy is preemption safe.

Will skip the test case when CONFIG_PREEMPT is disabled, and it can only
reproduce the problem probabilistically. By increasing
TASK_STORAGE_MAP_NR_LOOP and running it under ARM64 VM with 4-cpus, it
takes about four rounds to reproduce:

> test_maps is modified to only run test_task_storage_map_stress_lookup()
$ export TASK_STORAGE_MAP_NR_THREAD=256
$ export TASK_STORAGE_MAP_NR_LOOP=81920
$ export TASK_STORAGE_MAP_PIN_CPU=1
$ time ./test_maps
test_task_storage_map_stress_lookup(135):FAIL:bad bpf_task_storage_busy got -2

real    0m24.743s
user    0m6.772s
sys     0m17.966s

Signed-off-by: Hou Tao <houtao1@huawei.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/r/20220901061938.3789460-5-houtao@huaweicloud.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

1 files changed, 39 insertions, 0 deletions

diff --git a/tools/testing/selftests/bpf/progs/read_bpf_task_storage_busy.c b/tools/testing/selftests/bpf/progs/read_bpf_task_storage_busy.c
new file mode 100644
index 000000000000..a47bb0120719
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/read_bpf_task_storage_busy.c
@@ -0,0 +1,39 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (C) 2022. Huawei Technologies Co., Ltd */
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+extern bool CONFIG_PREEMPT __kconfig __weak;
+extern const int bpf_task_storage_busy __ksym;
+
+char _license[] SEC("license") = "GPL";
+
+int pid = 0;
+int busy = 0;
+
+struct {
+	__uint(type, BPF_MAP_TYPE_TASK_STORAGE);
+	__uint(map_flags, BPF_F_NO_PREALLOC);
+	__type(key, int);
+	__type(value, long);
+} task SEC(".maps");
+
+SEC("raw_tp/sys_enter")
+int BPF_PROG(read_bpf_task_storage_busy)
+{
+	int *value;
+	int key;
+
+	if (!CONFIG_PREEMPT)
+		return 0;
+
+	if (bpf_get_current_pid_tgid() >> 32 != pid)
+		return 0;
+
+	value = bpf_this_cpu_ptr(&bpf_task_storage_busy);
+	if (value)
+		busy = *value;
+
+	return 0;
+}