Re: [PATCH] percpu: fix race on alloc failed warning limit

["Christoph Lameter (Ampere)" <cl@gentwo.org>]

On Tue, 2 Sep 2025, Vlad Dumitrescu wrote:

> On 9/2/25 10:39, Christoph Lameter (Ampere) wrote:
> > On Fri, 22 Aug 2025, Vlad Dumitrescu wrote:
> >
> >> +	if (do_warn && atomic_read(&warn_limit) > 0) {
> >> +		int remaining = atomic_dec_return(&warn_limit);
> >
> >
> > The code creates a race condition since another atomic_dec_return() can
> > happen on another cpu between these two lines. warn_limit can go negative.
>
> Yes, which is why I mentioned it in the description. But compared to before,
> it should be benign.
>
> > Use a single atomic operation instead?
>
> Did you have something like this in mind?
>
> -	if (do_warn && atomic_read(&warn_limit) > 0) {
> -		int remaining = atomic_dec_return(&warn_limit);
> +	if (do_warn) {
> +		int remaining = atomic_dec_if_positive(&warn_limit);

Something like it... Maybe

if (do_warn && (atomic_dec_if_positive(&warn_limit)) ) {
  pr_warn  ...
}