selftests/bpf: Fix wq test.

The wq test was missing destroy(skel) part which was causing bpf progs to stay
loaded. That was causing test_progs to complain with
"Failed to unload bpf_testmod.ko from kernel: -11" message, but adding
destroy() wasn't enough, since wq callback may be delayed, so loop on unload of
bpf_testmod if errno is EAGAIN.

Acked-by: Andrii Nakryiko <andrii@kernel.org>
Fixes: 8290dba ("selftests/bpf: wq: add bpf_wq_start() checks")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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

@@ -31,6 +31,7 @@ void serial_test_wq(void)
 	usleep(50); /* 10 usecs should be enough, but give it extra */
 
 	ASSERT_EQ(wq_skel->bss->ok_sleepable, (1 << 1), "ok_sleepable");
+	wq__destroy(wq_skel);
 }
 
 void serial_test_failures_wq(void)

tools/testing/selftests/bpf/testing_helpers.c

@@ -368,9 +368,23 @@ int delete_module(const char *name, int flags)
 
 int unload_bpf_testmod(bool verbose)
 {
+	int ret, cnt = 0;
+
 	if (kern_sync_rcu())
 		fprintf(stdout, "Failed to trigger kernel-side RCU sync!\n");
-	if (delete_module("bpf_testmod", 0)) {
+
+	for (;;) {
+		ret = delete_module("bpf_testmod", 0);
+		if (!ret || errno != EAGAIN)
+			break;
+		if (++cnt > 10000) {
+			fprintf(stdout, "Unload of bpf_testmod timed out\n");
+			break;
+		}
+		usleep(100);
+	}
+
+	if (ret) {
 		if (errno == ENOENT) {
 			if (verbose)
 				fprintf(stdout, "bpf_testmod.ko is already unloaded.\n");