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.
