Re: [PATCH v4] md/raid5: fix atomicity violation in raid5_cache_count

[Song Liu]

On Thu, Jan 11, 2024 at 11:10 PM Gui-Dong Han <2045gemini@xxxxxxxxx> wrote:
>
[...]
>
>         raid5_release_stripe(sh);
> -       conf->max_nr_stripes++;
> +       WRITE_ONCE(conf->max_nr_stripes, conf->max_nr_stripes + 1);

This is weird. We are reading max_nr_stripes without READ_ONCE.

>         return 1;
>  }
>
> @@ -2707,7 +2707,7 @@ static int drop_one_stripe(struct r5conf *conf)
>         shrink_buffers(sh);
>         free_stripe(conf->slab_cache, sh);
>         atomic_dec(&conf->active_stripes);
> -       conf->max_nr_stripes--;
> +       WRITE_ONCE(conf->max_nr_stripes, conf->max_nr_stripes - 1);

Same here.

>         return 1;
>  }
>
> @@ -6820,7 +6820,7 @@ raid5_set_cache_size(struct mddev *mddev, int size)
>         if (size <= 16 || size > 32768)
>                 return -EINVAL;
>
> -       conf->min_nr_stripes = size;
> +       WRITE_ONCE(conf->min_nr_stripes, size);
>         mutex_lock(&conf->cache_size_mutex);
>         while (size < conf->max_nr_stripes &&
>                drop_one_stripe(conf))
> @@ -6832,7 +6832,7 @@ raid5_set_cache_size(struct mddev *mddev, int size)
>         mutex_lock(&conf->cache_size_mutex);
>         while (size > conf->max_nr_stripes)
>                 if (!grow_one_stripe(conf, GFP_KERNEL)) {
> -                       conf->min_nr_stripes = conf->max_nr_stripes;
> +                       WRITE_ONCE(conf->min_nr_stripes, conf->max_nr_stripes);

And here.

>                         result = -ENOMEM;
>                         break;
>                 }

Thanks,
Song