On Wed, May 22, 2024 at 07:30 PM +09, Tetsuo Handa wrote: > 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@xxxxxxxxx> >>>> On Sun, May 12, 2024 at 12:22=E2=80=AFAM Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> 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. You're right. My answer was too hasty. Docs say that RT kernels can preempt RCU read-side critical sections: https://docs.kernel.org/RCU/whatisRCU.html?highlight=rcu_read_lock#rcu-read-lock
