Re: [PATCH v3 bpf-next] bpf: Avoid unnecessary -EBUSY from htab_lock_bucket

[Song Liu <song@kernel.org>]

On Wed, Oct 4, 2023 at 11:32 PM Song Liu <song@kernel.org> wrote:
>
> htab_lock_bucket uses the following logic to avoid recursion:
>
> 1. preempt_disable();
> 2. check percpu counter htab->map_locked[hash] for recursion;
>    2.1. if map_lock[hash] is already taken, return -BUSY;
> 3. raw_spin_lock_irqsave();
>
> However, if an IRQ hits between 2 and 3, BPF programs attached to the IRQ
> logic will not able to access the same hash of the hashtab and get -EBUSY.
> This -EBUSY is not really necessary. Fix it by disabling IRQ before
> checking map_locked:
>
> 1. preempt_disable();
> 2. local_irq_save();
> 3. check percpu counter htab->map_locked[hash] for recursion;
>    3.1. if map_lock[hash] is already taken, return -BUSY;
> 4. raw_spin_lock().
>
> Similarly, use raw_spin_unlock() and local_irq_restore() in
> htab_unlock_bucket().
>
> Suggested-by: Tejun Heo <tj@kernel.org>
> Signed-off-by: Song Liu <song@kernel.org>

This still doesn't look right. Let me try more..

Thanks,
Song

>
> ---
> Changes in v3:
> 1. Use raw_local_irq_* APIs instead.
>
> Changes in v2:
> 1. Use raw_spin_unlock() and local_irq_restore() in htab_unlock_bucket().
>    (Andrii)
> ---
>  kernel/bpf/hashtab.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
> index a8c7e1c5abfa..74c8d1b41dd5 100644
> --- a/kernel/bpf/hashtab.c
> +++ b/kernel/bpf/hashtab.c
> @@ -155,13 +155,15 @@ static inline int htab_lock_bucket(const struct bpf_htab *htab,
>         hash = hash & min_t(u32, HASHTAB_MAP_LOCK_MASK, htab->n_buckets - 1);
>
>         preempt_disable();
> +       raw_local_irq_save(flags);
>         if (unlikely(__this_cpu_inc_return(*(htab->map_locked[hash])) != 1)) {
>                 __this_cpu_dec(*(htab->map_locked[hash]));
> +               raw_local_irq_restore(flags);
>                 preempt_enable();
>                 return -EBUSY;
>         }
>
> -       raw_spin_lock_irqsave(&b->raw_lock, flags);
> +       raw_spin_lock(&b->raw_lock);
>         *pflags = flags;
>
>         return 0;
> @@ -172,8 +174,9 @@ static inline void htab_unlock_bucket(const struct bpf_htab *htab,
>                                       unsigned long flags)
>  {
>         hash = hash & min_t(u32, HASHTAB_MAP_LOCK_MASK, htab->n_buckets - 1);
> -       raw_spin_unlock_irqrestore(&b->raw_lock, flags);
> +       raw_spin_unlock(&b->raw_lock);
>         __this_cpu_dec(*(htab->map_locked[hash]));
> +       raw_local_irq_restore(flags);
>         preempt_enable();
>  }
>
> --
> 2.34.1
>