Re: [PATCH 1/2] lib: fix compare_delta parameter order in percpu_counter_tree

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2026-03-16 10:38, David Carlier 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.
> 
> Signed-off-by: David Carlier <devnexen@gmail.com>

Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

> ---
>   lib/percpu_counter_tree.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/lib/percpu_counter_tree.c b/lib/percpu_counter_tree.c
> index 5c8fc2dcdc16..beb1144e6450 100644
> --- 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)


-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com