netdev - Re: [PATCH v4 bpf-next 09/14] bpf: Allow reuse from waiting_for_gp

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Date: Fri, 7 Jul 2023 11:38:52 +0800
From: Hou Tao <houtao@...weicloud.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Tejun Heo <tj@...nel.org>, rcu@...r.kernel.org,
 Network Development <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
 Kernel Team <kernel-team@...com>, Daniel Borkmann <daniel@...earbox.net>,
 Andrii Nakryiko <andrii@...nel.org>, David Vernet <void@...ifault.com>,
 "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH v4 bpf-next 09/14] bpf: Allow reuse from
 waiting_for_gp_ttrace list.

Hi,

On 7/7/2023 10:12 AM, Alexei Starovoitov wrote:
> On Thu, Jul 6, 2023 at 7:07 PM Hou Tao <houtao@...weicloud.com> wrote:
>> Hi,
>>
>> On 7/6/2023 11:34 AM, Alexei Starovoitov wrote:
>>> From: Alexei Starovoitov <ast@...nel.org>
>>>
>>> alloc_bulk() can reuse elements from free_by_rcu_ttrace.
>>> Let it reuse from waiting_for_gp_ttrace as well to avoid unnecessary kmalloc().
>>>
>>> Signed-off-by: Alexei Starovoitov <ast@...nel.org>
>>> ---
>>>  kernel/bpf/memalloc.c | 16 ++++++++++------
>>>  1 file changed, 10 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/kernel/bpf/memalloc.c b/kernel/bpf/memalloc.c
>>> index 9986c6b7df4d..e5a87f6cf2cc 100644
>>> --- a/kernel/bpf/memalloc.c
>>> +++ b/kernel/bpf/memalloc.c
>>> @@ -212,6 +212,15 @@ static void alloc_bulk(struct bpf_mem_cache *c, int cnt, int node)
>>>       if (i >= cnt)
>>>               return;
>>>
>>> +     for (; i < cnt; i++) {
>>> +             obj = llist_del_first(&c->waiting_for_gp_ttrace);
>>> +             if (!obj)
>>> +                     break;
>>> +             add_obj_to_free_list(c, obj);
>>> +     }
>>> +     if (i >= cnt)
>>> +             return;
>> I still think using llist_del_first() here is not safe as reported in
>> [1]. Not sure whether or not invoking enque_to_free() firstly for
>> free_llist_extra will close the race completely. Will check later.
> lol. see my reply a second ago in the other thread.
>
> and it's not just waiting_for_gp_ttrace. free_by_rcu_ttrace is similar.

I think free_by_rcu_ttrace is different, because the reuse is only
possible after one tasks trace RCU grace period as shown below, and the
concurrent llist_del_first() must have been completed when the head is
reused and re-added into free_by_rcu_ttrace again.

// c0->free_by_rcu_ttrace
A -> B -> C -> nil
   
P1:
alloc_bulk()
    llist_del_first(&c->free_by_rcu_ttrace)
        entry = A
        next = B

P2:
do_call_rcu_ttrace()
    // c->free_by_rcu_ttrace->first = NULL
    llist_del_all(&c->free_by_rcu_ttrace)
        move to c->waiting_for_gp_ttrace

P1:
llist_del_first()
    return NULL

// A is only reusable after one task trace RCU grace
// llist_del_first() must have been completed
__free_rcu_tasks_trace
    free_all(llist_del_all(&c->waiting_for_gp_ttrace))