selftests/bpf: Add test case for the freeing of bpf_timer

The main purpose of the test is to demonstrate the lock problem for the
free of bpf_timer under PREEMPT_RT. When freeing a bpf_timer which is
running on other CPU in bpf_timer_cancel_and_free(), hrtimer_cancel()
will try to acquire a spin-lock (namely softirq_expiry_lock), however
the freeing procedure has already held a raw-spin-lock.

The test first creates two threads: one to start timers and the other to
free timers. The start-timers thread will start the timer and then wake
up the free-timers thread to free these timers when the starts complete.
After freeing, the free-timer thread will wake up the start-timer thread
to complete the current iteration. A loop of 10 iterations is used.

Signed-off-by: Hou Tao <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

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

···
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (C) 2025. Huawei Technologies Co., Ltd */
+#define _GNU_SOURCE
+#include <unistd.h>
+#include <sys/syscall.h>
+#include <test_progs.h>
+
+#include "free_timer.skel.h"
+
+struct run_ctx {
+	struct bpf_program *start_prog;
+	struct bpf_program *overwrite_prog;
+	pthread_barrier_t notify;
+	int loop;
+	bool start;
+	bool stop;
+};
+
+static void start_threads(struct run_ctx *ctx)
+{
+	ctx->start = true;
+}
+
+static void stop_threads(struct run_ctx *ctx)
+{
+	ctx->stop = true;
+	/* Guarantee the order between ->stop and ->start */
+	__atomic_store_n(&ctx->start, true, __ATOMIC_RELEASE);
+}
+
+static int wait_for_start(struct run_ctx *ctx)
+{
+	while (!__atomic_load_n(&ctx->start, __ATOMIC_ACQUIRE))
+		usleep(10);
+
+	return ctx->stop;
+}
+
+static void *overwrite_timer_fn(void *arg)
+{
+	struct run_ctx *ctx = arg;
+	int loop, fd, err;
+	cpu_set_t cpuset;
+	long ret = 0;
+
+	/* Pin on CPU 0 */
+	CPU_ZERO(&cpuset);
+	CPU_SET(0, &cpuset);
+	pthread_setaffinity_np(pthread_self(), sizeof(cpuset), &cpuset);
+
+	/* Is the thread being stopped ? */
+	err = wait_for_start(ctx);
+	if (err)
+		return NULL;
+
+	fd = bpf_program__fd(ctx->overwrite_prog);
+	loop = ctx->loop;
+	while (loop-- > 0) {
+		LIBBPF_OPTS(bpf_test_run_opts, opts);
+
+		/* Wait for start thread to complete */
+		pthread_barrier_wait(&ctx->notify);
+
+		/* Overwrite timers */
+		err = bpf_prog_test_run_opts(fd, &opts);
+		if (err)
+			ret |= 1;
+		else if (opts.retval)
+			ret |= 2;
+
+		/* Notify start thread to start timers */
+		pthread_barrier_wait(&ctx->notify);
+	}
+
+	return (void *)ret;
+}
+
+static void *start_timer_fn(void *arg)
+{
+	struct run_ctx *ctx = arg;
+	int loop, fd, err;
+	cpu_set_t cpuset;
+	long ret = 0;
+
+	/* Pin on CPU 1 */
+	CPU_ZERO(&cpuset);
+	CPU_SET(1, &cpuset);
+	pthread_setaffinity_np(pthread_self(), sizeof(cpuset), &cpuset);
+
+	/* Is the thread being stopped ? */
+	err = wait_for_start(ctx);
+	if (err)
+		return NULL;
+
+	fd = bpf_program__fd(ctx->start_prog);
+	loop = ctx->loop;
+	while (loop-- > 0) {
+		LIBBPF_OPTS(bpf_test_run_opts, opts);
+
+		/* Run the prog to start timer */
+		err = bpf_prog_test_run_opts(fd, &opts);
+		if (err)
+			ret |= 4;
+		else if (opts.retval)
+			ret |= 8;
+
+		/* Notify overwrite thread to do overwrite */
+		pthread_barrier_wait(&ctx->notify);
+
+		/* Wait for overwrite thread to complete */
+		pthread_barrier_wait(&ctx->notify);
+	}
+
+	return (void *)ret;
+}
+
+void test_free_timer(void)
+{
+	struct free_timer *skel;
+	struct bpf_program *prog;
+	struct run_ctx ctx;
+	pthread_t tid[2];
+	void *ret;
+	int err;
+
+	skel = free_timer__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "open_load"))
+		return;
+
+	memset(&ctx, 0, sizeof(ctx));
+
+	prog = bpf_object__find_program_by_name(skel->obj, "start_timer");
+	if (!ASSERT_OK_PTR(prog, "find start prog"))
+		goto out;
+	ctx.start_prog = prog;
+
+	prog = bpf_object__find_program_by_name(skel->obj, "overwrite_timer");
+	if (!ASSERT_OK_PTR(prog, "find overwrite prog"))
+		goto out;
+	ctx.overwrite_prog = prog;
+
+	pthread_barrier_init(&ctx.notify, NULL, 2);
+	ctx.loop = 10;
+
+	err = pthread_create(&tid[0], NULL, start_timer_fn, &ctx);
+	if (!ASSERT_OK(err, "create start_timer"))
+		goto out;
+
+	err = pthread_create(&tid[1], NULL, overwrite_timer_fn, &ctx);
+	if (!ASSERT_OK(err, "create overwrite_timer")) {
+		stop_threads(&ctx);
+		goto out;
+	}
+
+	start_threads(&ctx);
+
+	ret = NULL;
+	err = pthread_join(tid[0], &ret);
+	ASSERT_EQ(err | (long)ret, 0, "start_timer");
+	ret = NULL;
+	err = pthread_join(tid[1], &ret);
+	ASSERT_EQ(err | (long)ret, 0, "overwrite_timer");
+out:
+	free_timer__destroy(skel);
+}

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

···
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (C) 2025. Huawei Technologies Co., Ltd */
+#include <linux/bpf.h>
+#include <time.h>
+#include <bpf/bpf_tracing.h>
+#include <bpf/bpf_helpers.h>
+
+#define MAX_ENTRIES 8
+
+struct map_value {
+	struct bpf_timer timer;
+};
+
+struct {
+	__uint(type, BPF_MAP_TYPE_HASH);
+	__type(key, int);
+	__type(value, struct map_value);
+	__uint(max_entries, MAX_ENTRIES);
+} map SEC(".maps");
+
+static int timer_cb(void *map, void *key, struct map_value *value)
+{
+	volatile int sum = 0;
+	int i;
+
+	bpf_for(i, 0, 1024 * 1024) sum += i;
+
+	return 0;
+}
+
+static int start_cb(int key)
+{
+	struct map_value *value;
+
+	value = bpf_map_lookup_elem(&map, (void *)&key);
+	if (!value)
+		return 0;
+
+	bpf_timer_init(&value->timer, &map, CLOCK_MONOTONIC);
+	bpf_timer_set_callback(&value->timer, timer_cb);
+	/* Hope 100us will be enough to wake-up and run the overwrite thread */
+	bpf_timer_start(&value->timer, 100000, BPF_F_TIMER_CPU_PIN);
+
+	return 0;
+}
+
+static int overwrite_cb(int key)
+{
+	struct map_value zero = {};
+
+	/* Free the timer which may run on other CPU */
+	bpf_map_update_elem(&map, (void *)&key, &zero, BPF_ANY);
+
+	return 0;
+}
+
+SEC("syscall")
+int BPF_PROG(start_timer)
+{
+	bpf_loop(MAX_ENTRIES, start_cb, NULL, 0);
+	return 0;
+}
+
+SEC("syscall")
+int BPF_PROG(overwrite_timer)
+{
+	bpf_loop(MAX_ENTRIES, overwrite_cb, NULL, 0);
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";