netdev - Re: [PATCH v2 bpf-next 12/13] bpf: Introduce bpf_mem_free_rcu() similar to kfree

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Date: Sat, 24 Jun 2023 14:49:04 +0800
From: Hou Tao <houtao@...weicloud.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>, daniel@...earbox.net,
 andrii@...nel.org, void@...ifault.com, paulmck@...nel.org
Cc: tj@...nel.org, rcu@...r.kernel.org, netdev@...r.kernel.org,
 bpf@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH v2 bpf-next 12/13] bpf: Introduce bpf_mem_free_rcu()
 similar to kfree_rcu().

Hi,

On 6/24/2023 11:13 AM, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@...nel.org>
>
SNIP
>  
> +static void __free_by_rcu(struct rcu_head *head)
> +{
> +	struct bpf_mem_cache *c = container_of(head, struct bpf_mem_cache, rcu);
> +	struct bpf_mem_cache *tgt = c->tgt;
> +	struct llist_node *llnode;
> +
> +	if (unlikely(READ_ONCE(c->draining)))
> +		goto out;
> +
> +	llnode = llist_del_all(&c->waiting_for_gp);
> +	if (!llnode)
> +		goto out;
> +
> +	if (llist_add_batch(llnode, c->waiting_for_gp_tail, &tgt->free_by_rcu_ttrace))
> +		tgt->free_by_rcu_ttrace_tail = c->waiting_for_gp_tail;
Got a null-ptr dereference oops when running multiple test_maps and
htab-mem benchmark after hacking htab to use bpf_mem_cache_free_rcu().
And I think it happened as follow:

// c->tgt
P1: __free_by_rcu()
        // c->tgt is the same as P1
        P2: __free_by_rcu()

// return true
P1: llist_add_batch(&tgt->free_by_rcu_ttrace)
        // return false
        P2: llist_add_batch(&tgt->free_by_rcu_ttrace)
        P2: do_call_rcu_ttrace
        // return false
        P2: xchg(tgt->call_rcu_ttrace_in_progress, 1)
        // llnode is not NULL
        P2: llnode = llist_del_all(&c->free_by_rcu_ttrace)
        // BAD: c->free_by_rcu_ttrace_tail is NULL, so oops
        P2: __llist_add_batch(llnode, c->free_by_rcu_ttrace_tail)

P1: tgt->free_by_rcu_ttrace_tail = X   

I don't have a good fix for the problem except adding a spin-lock for
free_by_rcu_ttrace and free_by_rcu_ttrace_tail.