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

[Tonghao Zhang]

On Wed, Nov 30, 2022 at 11:07 AM Waiman Long <longman@xxxxxxxxxx> wrote:
>
> On 11/29/22 21:47, Tonghao Zhang wrote:
> > On Wed, Nov 30, 2022 at 9:50 AM Hou Tao <houtao@xxxxxxxxxxxxxxx> 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@xxxxxxxxx> 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.
Yes, thanks Waiman, I should add the busy_out label.
> >          } 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.
there are some case, for example, we run the bpf_prog A B in task
context on the same cpu.
bpf_prog A
update map X
    htab_lock_bucket
        raw_spin_lock_irqsave()
    lookup_elem_raw()
        // bpf prog B is attached on lookup_elem_raw()
        bpf prog B
            update map X again and update the element
                htab_lock_bucket()
                    // dead-lock
                    raw_spinlock_irqsave()
> Cheers,
> Longman
>


-- 
Best regards, Tonghao