Re: [RFC PATCH v2 03/14] kcov: add barriers to recursion guard in kcov_df_write

[Alexander Potapenko]

On Thu, Jun 11, 2026 at 6:21 PM Yunseong Kim <yunseong.kim@xxxxxxxx> wrote:
>
> The recursion guard (bit-31 of kcov_df_seq) prevents reentry when
> copy_from_kernel_nofault() or other called functions are instrumented
> with INSTRUMENT_ALL. Without compiler barriers, the guard set/clear
> can be reordered relative to the function body, making the protection
> ineffective under optimization.
>
> Add barrier() after setting the guard and before clearing it, ensuring
> the compiler does not move instrumented operations outside the guarded
> region.
>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Yunseong Kim <yunseong.kim@xxxxxxxx>
> ---
>  kernel/kcov_dataflow.c | 2 ++

Please merge this patch into the one introducing kcov_dataflow.c


>  1 file changed, 2 insertions(+)
>
> diff --git a/kernel/kcov_dataflow.c b/kernel/kcov_dataflow.c
> index df7e8bf70bfa..5248293280d5 100644
> --- a/kernel/kcov_dataflow.c
> +++ b/kernel/kcov_dataflow.c
> @@ -86,6 +86,7 @@ kcov_df_write(u64 type_marker, u64 pc, u64 meta, void *ptr,
>         if (t->kcov_df_seq & (1U << 31))
>                 return;
>         t->kcov_df_seq |= (1U << 31);
> +       barrier();

Please make sure barriers have comments explaining which barriers they
pair with (see kernel/kcov.c)