selftests/bpf: Fix potential premature unload in bpf_testmod

It is possible for bpf_kfunc_call_test_release() to be called from
bpf_map_free_deferred() when bpf_testmod is already unloaded and
perf_test_stuct.cnt which it tries to decrease is no longer in memory.
This patch tries to fix the issue by waiting for all references to be
dropped in bpf_testmod_exit().

The issue can be triggered by running 'test_progs -t map_kptr' in 6.5,
but is obscured in 6.6 by d119357d ("rcu-tasks: Treat only
synchronous grace periods urgently").

Fixes: 65eb006d ("bpf: Move kernel test kfuncs to bpf_testmod")
Signed-off-by: Artem Savkov <asavkov@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Cc: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/82f55c0e-0ec8-4fe1-8d8c-b1de07558ad9@linux.dev
Link: https://lore.kernel.org/bpf/20240110085737.8895-1-asavkov@redhat.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c

@@ -2,6 +2,7 @@
 /* Copyright (c) 2020 Facebook */
 #include <linux/btf.h>
 #include <linux/btf_ids.h>
+#include <linux/delay.h>
 #include <linux/error-injection.h>
 #include <linux/init.h>
 #include <linux/module.h>
@@ -544,6 +545,14 @@ static int bpf_testmod_init(void)
 
 static void bpf_testmod_exit(void)
 {
+        /* Need to wait for all references to be dropped because
+         * bpf_kfunc_call_test_release() which currently resides in kernel can
+         * be called after bpf_testmod is unloaded. Once release function is
+         * moved into the module this wait can be removed.
+         */
+	while (refcount_read(&prog_test_struct.cnt) > 1)
+		msleep(20);
+
 	return sysfs_remove_bin_file(kernel_kobj, &bin_attr_bpf_testmod_file);
 }