Commit eb814cf1 authored by Delyan Kratunov's avatar Delyan Kratunov Committed by Alexei Starovoitov
Browse files

selftests/bpf: fix task_local_storage/exit_creds rcu usage 

BPF CI has revealed flakiness in the task_local_storage/exit_creds test.
The failure point in CI [1] is that null_ptr_count is equal to 0,
which indicates that the program hasn't run yet. This points to the
kern_sync_rcu (sys_membarrier -> synchronize_rcu underneath) not
waiting sufficiently.

Indeed, synchronize_rcu only waits for read-side sections that started
before the call. If the program execution starts *during* the
synchronize_rcu invocation (due to, say, preemption), the test won't
wait long enough.

As a speculative fix, make the synchornize_rcu calls in a loop until
an explicit run counter has gone up.

  [1]: https://github.com/kernel-patches/bpf/actions/runs/3268263235/jobs/5374940791



Signed-off-by: default avatarDelyan Kratunov <delyank@meta.com>
Link: https://lore.kernel.org/r/156d4ef82275a074e8da8f4cffbd01b0c1466493.camel@meta.com


Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
parent 12f96823

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

+15 −3
Original line number Diff line number Diff line
@@ -39,7 +39,8 @@ static void test_sys_enter_exit(void) 
static void test_exit_creds(void)

{

	struct task_local_storage_exit_creds *skel;

	int err;

	int err, run_count, sync_rcu_calls = 0;

	const int MAX_SYNC_RCU_CALLS = 1000;



	skel = task_local_storage_exit_creds__open_and_load();

	if (!ASSERT_OK_PTR(skel, "skel_open_and_load"))
@@ -53,8 +54,19 @@ static void test_exit_creds(void) 
	if (CHECK_FAIL(system("ls > /dev/null")))

		goto out;



	/* sync rcu to make sure exit_creds() is called for "ls" */

	/* kern_sync_rcu is not enough on its own as the read section we want

	 * to wait for may start after we enter synchronize_rcu, so our call

	 * won't wait for the section to finish. Loop on the run counter

	 * as well to ensure the program has run.

	 */

	do {

		kern_sync_rcu();

		run_count = __atomic_load_n(&skel->bss->run_count, __ATOMIC_SEQ_CST);

	} while (run_count == 0 && ++sync_rcu_calls < MAX_SYNC_RCU_CALLS);



	ASSERT_NEQ(sync_rcu_calls, MAX_SYNC_RCU_CALLS,

		   "sync_rcu count too high");

	ASSERT_NEQ(run_count, 0, "run_count");

	ASSERT_EQ(skel->bss->valid_ptr_count, 0, "valid_ptr_count");

	ASSERT_NEQ(skel->bss->null_ptr_count, 0, "null_ptr_count");

out:

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

+3 −0
Original line number Diff line number Diff line
@@ -14,6 +14,7 @@ struct { 
	__type(value, __u64);

} task_storage SEC(".maps");



int run_count = 0;

int valid_ptr_count = 0;

int null_ptr_count = 0;
@@ -28,5 +29,7 @@ int BPF_PROG(trace_exit_creds, struct task_struct *task) 
		__sync_fetch_and_add(&valid_ptr_count, 1);

	else

		__sync_fetch_and_add(&null_ptr_count, 1);



	__sync_fetch_and_add(&run_count, 1);

	return 0;

}