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

[Josh Law]

On 13 March 2026 17:54:24 GMT, 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.
>
>Signed-off-by: David Carlier <devnexen@xxxxxxxxx>
>---
> 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)


Good patch.


Reviewed-by: Josh Law <objecting@xxxxxxxxxxxxx>


V/R


Josh Law