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

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Date: Tue, 27 Jun 2023 17:54:14 -0700
From: Alexei Starovoitov <ast@...a.com>
To: Hou Tao <houtao@...weicloud.com>,
        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().

On 6/24/23 12:53 AM, Hou Tao wrote:
> Hi,
> 
> On 6/24/2023 11:13 AM, Alexei Starovoitov wrote:
>> From: Alexei Starovoitov <ast@...nel.org>
>>
>> Introduce bpf_mem_[cache_]free_rcu() similar to kfree_rcu().
>> Unlike bpf_mem_[cache_]free() that links objects for immediate reuse into
>> per-cpu free list the _rcu() flavor waits for RCU grace period and then moves
>> objects into free_by_rcu_ttrace list where they are waiting for RCU
>> task trace grace period to be freed into slab.
>>
> SNIP
>> +static void check_free_by_rcu(struct bpf_mem_cache *c)
>> +{
>> +	struct llist_node *llnode, *t;
>> +
>> +	if (llist_empty(&c->free_by_rcu) && llist_empty(&c->free_llist_extra_rcu))
>> +		return;
>> +
>> +	/* drain free_llist_extra_rcu */
>> +	llist_for_each_safe(llnode, t, llist_del_all(&c->free_llist_extra_rcu))
>> +		if (__llist_add(llnode, &c->free_by_rcu))
>> +			c->free_by_rcu_tail = llnode;
> 
> Just like add_obj_to_free_list(),  we should do conditional
> local_irq_save(flags) and local_inc_return(&c->active) as well for
> free_by_rcu, otherwise free_by_rcu may be corrupted by bpf program
> running in a NMI context.

Good catch. Will do.