netdev - Re: [PATCH net-next v2 04/12] net: sched: flower: track filter deletion with flag

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Date:   Sat, 2 Mar 2019 00:51:29 +0100
From:   Stefano Brivio <sbrivio@...hat.com>
To:     Vlad Buslov <vladbu@...lanox.com>
Cc:     netdev@...r.kernel.org, jhs@...atatu.com, xiyou.wangcong@...il.com,
        jiri@...nulli.us, davem@...emloft.net
Subject: Re: [PATCH net-next v2 04/12] net: sched: flower: track filter
 deletion with flag

Hi Vlad,

On Wed, 27 Feb 2019 12:12:18 +0200
Vlad Buslov <vladbu@...lanox.com> wrote:

> -static bool __fl_delete(struct tcf_proto *tp, struct cls_fl_filter *f,
> -			struct netlink_ext_ack *extack)
> +static int __fl_delete(struct tcf_proto *tp, struct cls_fl_filter *f,
> +		       bool *last, struct netlink_ext_ack *extack)
>  {
>  	struct cls_fl_head *head = fl_head_dereference(tp);
>  	bool async = tcf_exts_get_net(&f->exts);
> -	bool last;
>  
> +	*last = false;
> +
> +	if (f->deleted)
> +		return -ENOENT;
> +
> +	f->deleted = true;

Now that I can read this more easily :) I have a doubt: you say this
flag "prevent[s] double deletion of filter by concurrent tasks".

However, if this has no further protections (which I can't readily
see), I think this is racy:

task 1				task 2
if (f->deleted) [false]
				if (f->deleted) [false]
f->deleted = true;		f->deleted = true;

what am I missing here?

-- 
Stefano