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

[Tonghao Zhang]

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;
        } else {
                raw_spin_lock_irqsave(&b->raw_lock, flags);
        }

        *pflags = flags;
        return 0;
}


> lockdep deduces that may be a dead-lock. I have already tried to use the same
> map_locked for keys with the same bucket, the dead-lock is gone, but still got
> lockdep warning.
> >
> > Hao
> > .
>


-- 
Best regards, Tonghao