Re: [PATCH] mm: vmscan: make unregister_shrinker() safer

[Michal Hocko <mhocko@kernel.org>]

On Sat 16-12-17 22:29:37, Aliaksei Karaliou wrote:
> unregister_shrinker() does not have any sanitizing inside so
> calling it twice will oops because of double free attempt or so.
> This patch makes unregister_shrinker() safer and allows calling
> it on resource freeing path without explicit knowledge of whether
> shrinker was successfully registered or not.

Tetsuo has made it half way to this already [1]. So maybe we should
fold shrinker->nr_deferred = NULL to his patch and finally merge it.

[1] http://lkml.kernel.org/r/1511523385-6433-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp
> 
> Signed-off-by: Aliaksei Karaliou <akaraliou.dev@gmail.com>
> ---
>  mm/vmscan.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 65c4fa26abfa..7cb56db5e9ca 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -281,10 +281,14 @@ EXPORT_SYMBOL(register_shrinker);
>   */
>  void unregister_shrinker(struct shrinker *shrinker)
>  {
> +	if (!shrinker->nr_deferred)
> +		return;
> +
>  	down_write(&shrinker_rwsem);
>  	list_del(&shrinker->list);
>  	up_write(&shrinker_rwsem);
>  	kfree(shrinker->nr_deferred);
> +	shrinker->nr_deferred = NULL;
>  }
>  EXPORT_SYMBOL(unregister_shrinker);
>  
> -- 
> 2.11.0
> 

-- 
Michal Hocko
SUSE Labs

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