linux-kernel - Re: [PATCH 1/2] ucounts: Move kfree() out of critical zone protected by ucounts

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Message-Id: <1733458077-11339-1-git-send-email-mengensun@tencent.com>
Date: Fri,  6 Dec 2024 12:07:57 +0800
From: MengEn Sun <mengensun88@...il.com>
To: mengensun88@...il.com
Cc: akpm@...ux-foundation.org,
	avagin@...gle.com,
	joel.granados@...nel.org,
	linux-kernel@...r.kernel.org,
	linux@...ssschuh.net,
	mengensun@...cent.com,
	yuehongwu@...cent.com
Subject: Re: [PATCH 1/2] ucounts: Move kfree() out of critical zone protected by ucounts_lock 

I'm very sorry to disturb everyone. I wrote the subject incorrectly, so
this one is void; I will send a new one.

> Date: Fri,  6 Dec 2024 10:57:22 +0800	[thread overview]
> Message-ID: <1733453842-5773-1-git-send-email-mengensun@...cent.com> (raw)
> 
> From: MengEn Sun <mengensun@...cent.com>
> 
> Although kfree is a non-sleep function, it is possible to enter a
> long chain of calls probabilistically, so it looks better to move
> kfree from alloc_ucounts() out of the critical zone of ucounts_lock.
> 
> Reviewed-by: YueHong Wu <yuehongwu@...cent.com>
> Signed-off-by: MengEn Sun <mengensun@...cent.com>
> ---
>  kernel/ucount.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/ucount.c b/kernel/ucount.c
> index f950b5e5..86c5f1c 100644
> --- a/kernel/ucount.c
> +++ b/kernel/ucount.c
> @@ -164,8 +164,8 @@ struct ucounts *get_ucounts(struct ucounts *ucounts)
>  struct ucounts *alloc_ucounts(struct user_namespace *ns, kuid_t uid)
>  {
>  	struct hlist_head *hashent = ucounts_hashentry(ns, uid);
> -	struct ucounts *ucounts, *new;
>  	bool wrapped;
> +	struct ucounts *ucounts, *new = NULL;
>  
>  	spin_lock_irq(&ucounts_lock);
>  	ucounts = find_ucounts(ns, uid, hashent);
> @@ -182,17 +182,17 @@ struct ucounts *alloc_ucounts(struct
> user_namespace *ns, kuid_t uid)
>  
>  		spin_lock_irq(&ucounts_lock);
>  		ucounts = find_ucounts(ns, uid, hashent);
> -		if (ucounts) {
> -			kfree(new);
> -		} else {
> +		if (!ucounts) {
>  			hlist_add_head(&new->node, hashent);
>  			get_user_ns(new->ns);
>  			spin_unlock_irq(&ucounts_lock);
>  			return new;
>  		}
>  	}
> +
>  	wrapped = !get_ucounts_or_wrap(ucounts);
>  	spin_unlock_irq(&ucounts_lock);
> +	kfree(new);
>  	if (wrapped) {
>  		put_ucounts(ucounts);
>  		return NULL;
> --