Re: [PATCH 1/1] rcu: fix shrink budget underflow in lazy_rcu_shrink_scan

[Paul E. McKenney]

On Mon, Aug 24, 2026 at 09:31:42PM +0800, Longlong Xia wrote:
> From: Longlong Xia <xialonglong@xxxxxxxxxx>
> 
> The lazy RCU shrinker decremented sc->nr_to_scan (unsigned long)
> and then tested the result with <= 0.  When a single CPU's lazy
> callback count exceeds the remaining budget, the subtraction wraps
> to a large positive value and the <= 0 comparison, which is
> equivalent to == 0 for an unsigned type, never fires again.  The
> scan loop then iterates through every nocb CPU instead of honouring
> the reclaim budget.
> 
> Reorder the logic to compare _count against nr_to_scan before
> subtracting, so the loop exits as soon as the budget is met.
> 
> Fixes: c945b4da7a448 ("rcu: Shrinker for lazy rcu")
> Assisted-by: Zcode:GLM-5.2
> Signed-off-by: Longlong Xia <xialonglong@xxxxxxxxxx>

Good catch, and plausible fix.  Queued for further review and testing.

Frederic, thoughts?

							Thanx, Paul

> ---
>  kernel/rcu/tree_nocb.h | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index 19bb42672baf..afc3ab84cc47 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -1332,10 +1332,11 @@ lazy_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
>  		rcu_nocb_try_flush_bypass(rdp, jiffies);
>  		rcu_nocb_unlock_irqrestore(rdp, flags);
>  		wake_nocb_gp(rdp);
> -		sc->nr_to_scan -= _count;
>  		count += _count;
> -		if (sc->nr_to_scan <= 0)
> +
> +		if (_count >= sc->nr_to_scan)
>  			break;
> +		sc->nr_to_scan -= _count;
>  	}
>  
>  	mutex_unlock(&rcu_state.nocb_mutex);
> -- 
> 2.43.0
>