Re: [PATCH] lib: fix compare_delta parameter order in percpu_counter_tree

[Andrew Morton]

On Fri, 13 Mar 2026 17:54:24 +0000 David Carlier <devnexen@xxxxxxxxx> wrote:

> The compare_delta() helper takes (delta, accuracy_neg, accuracy_pos),
> but every call site passes (delta, accuracy_pos, accuracy_neg) — the
> last two arguments are consistently swapped.
> 
> The documented invariant (include/linux/percpu_counter_tree.h) is:
> 
>   (precise_sum - under) <= approx_sum <= (precise_sum + over)
> 
> Which means precise_sum is in [approx_sum - over, approx_sum + under].
> 
> For a positive delta (v - approx_sum >= 0), accuracy_pos must be
> "under" (the maximum amount precise_sum can exceed approx_sum).
> For a negative delta, accuracy_neg must be "over". Since under > over
> always (batch_size * M vs (batch_size - 1) * M), swapping them causes
> false definitive results: the functions return 1 ("v > counter") when
> the correct answer is 0 (indeterminate).
> 
> This affects all comparison functions:
> - percpu_counter_tree_approximate_compare_value()
> - percpu_counter_tree_approximate_compare()
> - percpu_counter_tree_precise_compare_value()
> - percpu_counter_tree_precise_compare()
> 
> The precise variants are also affected because their approximate
> fast-path can short-circuit with a wrong result, skipping the precise
> sum computation.
> 
> Fix by swapping the parameter order in compare_delta() itself, since
> all call sites are consistently swapped.

This affects mm-unstable's "lib: introduce hierarchical per-cpu
counters", so let's cc Mathieu.

> --- a/lib/percpu_counter_tree.c
> +++ b/lib/percpu_counter_tree.c
> @@ -458,7 +458,7 @@ long percpu_counter_tree_precise_sum(struct percpu_counter_tree *counter)
>  EXPORT_SYMBOL_GPL(percpu_counter_tree_precise_sum);
>  
>  static
> -int compare_delta(long delta, unsigned long accuracy_neg, unsigned long accuracy_pos)
> +int compare_delta(long delta, unsigned long accuracy_pos, unsigned long accuracy_neg)
>  {
>  	if (delta >= 0) {
>  		if (delta <= accuracy_pos)