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: Delyan Kratunov <delyank@xxxxxx> --- v1 -> v2: Explicit loop counter and MAX_SYNC_RCU_CALLS guard. .../bpf/prog_tests/task_local_storage.c | 18 +++++++++++++++--- .../bpf/progs/task_local_storage_exit_creds.c | 3 +++ 2 files changed, 18 insertions(+), 3 deletions(-) diff --git a/tools/testing/selftests/bpf/prog_tests/task_local_storage.c b/tools/testing/selftests/bpf/prog_tests/task_local_storage.c index 035c263aab1b..99a42a2b6e14 100644 --- a/tools/testing/selftests/bpf/prog_tests/task_local_storage.c +++ b/tools/testing/selftests/bpf/prog_tests/task_local_storage.c @@ -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(); + /* 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: diff --git a/tools/testing/selftests/bpf/progs/task_local_storage_exit_creds.c b/tools/testing/selftests/bpf/progs/task_local_storage_exit_creds.c index 81758c0aef99..41d88ed222ff 100644 --- a/tools/testing/selftests/bpf/progs/task_local_storage_exit_creds.c +++ b/tools/testing/selftests/bpf/progs/task_local_storage_exit_creds.c @@ -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; } -- 2.37.3
