Re: [net-next] bpf: avoid hashtab deadlock with try_lock

[Waiman Long <longman@redhat.com>]

On 11/29/22 21:47, Tonghao Zhang wrote:
> On Wed, Nov 30, 2022 at 9:50 AM Hou Tao <houtao@huaweicloud.com> wrote:
>> Hi Hao,
>>
>> On 11/30/2022 3:36 AM, Hao Luo wrote:
>>> On Tue, Nov 29, 2022 at 9:32 AM Boqun Feng <boqun.feng@gmail.com> wrote:
>>>> Just to be clear, I meant to refactor htab_lock_bucket() into a try
>>>> lock pattern. Also after a second thought, the below suggestion doesn't
>>>> work. I think the proper way is to make htab_lock_bucket() as a
>>>> raw_spin_trylock_irqsave().
>>>>
>>>> Regards,
>>>> Boqun
>>>>
>>> The potential deadlock happens when the lock is contended from the
>>> same cpu. When the lock is contended from a remote cpu, we would like
>>> the remote cpu to spin and wait, instead of giving up immediately. As
>>> this gives better throughput. So replacing the current
>>> raw_spin_lock_irqsave() with trylock sacrifices this performance gain.
>>>
>>> I suspect the source of the problem is the 'hash' that we used in
>>> htab_lock_bucket(). The 'hash' is derived from the 'key', I wonder
>>> whether we should use a hash derived from 'bucket' rather than from
>>> 'key'. For example, from the memory address of the 'bucket'. Because,
>>> different keys may fall into the same bucket, but yield different
>>> hashes. If the same bucket can never have two different 'hashes' here,
>>> the map_locked check should behave as intended. Also because
>>> ->map_locked is per-cpu, execution flows from two different cpus can
>>> both pass.
>> The warning from lockdep is due to the reason the bucket lock A is used in a
>> no-NMI context firstly, then the same bucke lock is used a NMI context, so
> Yes, I tested lockdep too, we can't use the lock in NMI(but only
> try_lock work fine) context if we use them no-NMI context. otherwise
> the lockdep prints the warning.
> * for the dead-lock case: we can use the
> 1. hash & min(HASHTAB_MAP_LOCK_MASK, htab->n_buckets -1)
> 2. or hash bucket address.
>
> * for lockdep warning, we should use in_nmi check with map_locked.
>
> BTW, the patch doesn't work, so we can remove the lock_key
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c50eb518e262fa06bd334e6eec172eaf5d7a5bd9
>
> static inline int htab_lock_bucket(const struct bpf_htab *htab,
>                                     struct bucket *b, u32 hash,
>                                     unsigned long *pflags)
> {
>          unsigned long flags;
>
>          hash = hash & min(HASHTAB_MAP_LOCK_MASK, htab->n_buckets -1);
>
>          preempt_disable();
>          if (unlikely(__this_cpu_inc_return(*(htab->map_locked[hash])) != 1)) {
>                  __this_cpu_dec(*(htab->map_locked[hash]));
>                  preempt_enable();
>                  return -EBUSY;
>          }
>
>          if (in_nmi()) {
>                  if (!raw_spin_trylock_irqsave(&b->raw_lock, flags))
>                          return -EBUSY;
That is not right. You have to do the same step as above by decrementing 
the percpu count and enable preemption. So you may want to put all these 
busy_out steps after the return 0 and use "goto busy_out;" to jump there.
>          } else {
>                  raw_spin_lock_irqsave(&b->raw_lock, flags);
>          }
>
>          *pflags = flags;
>          return 0;
> }

BTW, with that change, I believe you can actually remove all the percpu 
map_locked count code.

Cheers,
Longman