Re: [PATCH v3 2/3] perf lock contention: Check race in tstamp elem creation

[Ian Rogers]

On Fri, Oct 20, 2023 at 1:47 PM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
>
> When pelem is NULL, it'd create a new entry with zero data.  But it
> might be preempted by IRQ/NMI just before calling bpf_map_update_elem()
> then there's a chance to call it twice for the same pid.  So it'd be
> better to use BPF_NOEXIST flag and check the return value to prevent
> the race.
>
> Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>

Acked-by: Ian Rogers <irogers@xxxxxxxxxx>

Thanks,
Ian
> ---
>  tools/perf/util/bpf_skel/lock_contention.bpf.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/tools/perf/util/bpf_skel/lock_contention.bpf.c b/tools/perf/util/bpf_skel/lock_contention.bpf.c
> index b11179452e19..69d31fd77cd0 100644
> --- a/tools/perf/util/bpf_skel/lock_contention.bpf.c
> +++ b/tools/perf/util/bpf_skel/lock_contention.bpf.c
> @@ -328,7 +328,11 @@ int contention_begin(u64 *ctx)
>         if (pelem == NULL) {
>                 struct tstamp_data zero = {};
>
> -               bpf_map_update_elem(&tstamp, &pid, &zero, BPF_ANY);
> +               if (bpf_map_update_elem(&tstamp, &pid, &zero, BPF_NOEXIST) < 0) {
> +                       __sync_fetch_and_add(&task_fail, 1);
> +                       return 0;
> +               }
> +
>                 pelem = bpf_map_lookup_elem(&tstamp, &pid);
>                 if (pelem == NULL) {
>                         __sync_fetch_and_add(&task_fail, 1);
> --
> 2.42.0.655.g421f12c284-goog
>