selftests/bpf: Add a testcase for deadlock avoidance

Add a testcase that checks that deadlock avoidance is working
as expected.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20260204055147.54960-3-alexei.starovoitov@gmail.com

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

@@ -0,0 +1,75 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 Meta Platforms, Inc. and affiliates. */
+#include <vmlinux.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+#define CLOCK_MONOTONIC 1
+
+char _license[] SEC("license") = "GPL";
+
+struct elem {
+	struct bpf_timer timer;
+};
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__uint(max_entries, 1);
+	__type(key, int);
+	__type(value, struct elem);
+} timer_map SEC(".maps");
+
+volatile int in_timer_start;
+volatile int tp_called;
+
+static int timer_cb(void *map, int *key, struct elem *value)
+{
+	return 0;
+}
+
+SEC("tp_btf/hrtimer_cancel")
+int BPF_PROG(tp_hrtimer_cancel, struct hrtimer *hrtimer)
+{
+	struct bpf_timer *timer;
+	static bool called = false;
+	int key = 0;
+
+	if (!in_timer_start)
+		return 0;
+
+	tp_called = 1;
+	timer = bpf_map_lookup_elem(&timer_map, &key);
+
+	/*
+	 * Call bpf_timer_start() from the tracepoint within hrtimer logic
+	 * on the same timer to make sure it doesn't deadlock,
+	 * and do it once.
+	 */
+	if (!called) {
+		called = true;
+		bpf_timer_start(timer, 1000000000, 0);
+	}
+	return 0;
+}
+
+SEC("syscall")
+int start_timer(void *ctx)
+{
+	struct bpf_timer *timer;
+	int key = 0;
+
+	timer = bpf_map_lookup_elem(&timer_map, &key);
+	/* claude may complain here that there is no NULL check. Ignoring it. */
+	bpf_timer_init(timer, &timer_map, CLOCK_MONOTONIC);
+	bpf_timer_set_callback(timer, timer_cb);
+
+	/*
+	 * call hrtimer_start() twice, so that 2nd call does
+	 * remove_hrtimer() and trace_hrtimer_cancel() tracepoint.
+	 */
+	in_timer_start = 1;
+	bpf_timer_start(timer, 1000000000, 0);
+	bpf_timer_start(timer, 1000000000, 0);
+	in_timer_start = 0;
+	return 0;
+}