linux-kernel - Re: [PATCH] bpf, sockmap: defer sk_psock_free

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Date: Wed, 22 May 2024 19:30:58 +0900
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To: Jakub Sitnicki <jakub@...udflare.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Eric Dumazet <edumazet@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        Hillf Danton <hdanton@...a.com>,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH] bpf, sockmap: defer sk_psock_free_link() using RCU

On 2024/05/22 18:50, Jakub Sitnicki wrote:
> On Wed, May 22, 2024 at 06:59 AM +08, Hillf Danton wrote:
>> On Tue, 21 May 2024 08:38:52 -0700 Alexei Starovoitov <alexei.starovoitov@...il.com>
>>> On Sun, May 12, 2024 at 12:22=E2=80=AFAM Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp> wrote:
>>>> --- a/net/core/sock_map.c
>>>> +++ b/net/core/sock_map.c
>>>> @@ -142,6 +142,7 @@ static void sock_map_del_link(struct sock *sk,
>>>>         bool strp_stop =3D false, verdict_stop =3D false;
>>>>         struct sk_psock_link *link, *tmp;
>>>>
>>>> +       rcu_read_lock();
>>>>         spin_lock_bh(&psock->link_lock);
>>>
>>> I think this is incorrect.
>>> spin_lock_bh may sleep in RT and it won't be safe to do in rcu cs.
>>
>> Could you specify why it won't be safe in rcu cs if you are right?
>> What does rcu look like in RT if not nothing?
> 
> RCU readers can't block, while spinlock RT doesn't disable preemption.
> 
> https://docs.kernel.org/RCU/rcu.html
> https://docs.kernel.org/locking/locktypes.html#spinlock-t-and-preempt-rt
> 

I didn't catch what you mean.

https://elixir.bootlin.com/linux/latest/source/include/linux/spinlock_rt.h#L43 defines spin_lock() for RT as

static __always_inline void spin_lock(spinlock_t *lock)
{
	rt_spin_lock(lock);
}

and https://elixir.bootlin.com/linux/v6.9/source/include/linux/spinlock_rt.h#L85 defines spin_lock_bh() for RT as

static __always_inline void spin_lock_bh(spinlock_t *lock)
{
	/* Investigate: Drop bh when blocking ? */
	local_bh_disable();
	rt_spin_lock(lock);
}

and https://elixir.bootlin.com/linux/latest/source/kernel/locking/spinlock_rt.c#L54 defines rt_spin_lock() for RT as

void __sched rt_spin_lock(spinlock_t *lock)
{
	spin_acquire(&lock->dep_map, 0, 0, _RET_IP_);
	__rt_spin_lock(lock);
}

and https://elixir.bootlin.com/linux/v6.9/source/kernel/locking/spinlock_rt.c#L46 defines __rt_spin_lock() for RT as

static __always_inline void __rt_spin_lock(spinlock_t *lock)
{
	rtlock_might_resched();
	rtlock_lock(&lock->lock);
	rcu_read_lock();
	migrate_disable();
}

 You can see that calling spin_lock() or spin_lock_bh() automatically starts RCU critical section, can't you?

If spin_lock_bh() for RT might sleep and calling spin_lock_bh() under RCU critical section is not safe,
how can

  spin_lock(&lock1);
  spin_lock(&lock2);
  // do something
  spin_unlock(&lock2);
  spin_unlock(&lock1);

or

  spin_lock_bh(&lock1);
  spin_lock(&lock2);
  // do something
  spin_unlock(&lock2);
  spin_unlock_bh(&lock1);

be possible?

Unless rcu_read_lock() is implemented in a way that is safe to do

  rcu_read_lock();
  spin_lock(&lock2);
  // do something
  spin_unlock(&lock2);
  rcu_read_unlock();

and

  rcu_read_lock();
  spin_lock_bh(&lock2);
  // do something
  spin_unlock_bh(&lock2);
  rcu_read_unlock();

, I think RT kernels can't run safely.

Locking primitive ordering is too much complicated/distributed.
We need documentation using safe/unsafe ordering examples.