Re: [PATCH v2 bpf-next] bpf: Avoid unnecessary -EBUSY from htab_lock_bucket

[Song Liu <songliubraving@meta.com>]

> On Oct 3, 2023, at 8:33 PM, Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:
> 
> On Tue, Oct 3, 2023 at 8:08 PM Andrii Nakryiko
> <andrii.nakryiko@gmail.com> wrote:
>> 
>> On Tue, Oct 3, 2023 at 5:45 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>
>>> 
>>> ---
>>> 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(-)
>>> 
>> 
>> Now it's more symmetrical and seems correct to me, thanks!
>> 
>> Acked-by: Andrii Nakryiko <andrii@kernel.org>
>> 
>>> diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
>>> index a8c7e1c5abfa..fd8d4b0addfc 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();
>>> +       local_irq_save(flags);
>>>        if (unlikely(__this_cpu_inc_return(*(htab->map_locked[hash])) != 1)) {
>>>                __this_cpu_dec(*(htab->map_locked[hash]));
>>> +               local_irq_restore(flags);
>>>                preempt_enable();
>>>                return -EBUSY;
>>>        }
>>> 
>>> -       raw_spin_lock_irqsave(&b->raw_lock, flags);
>>> +       raw_spin_lock(&b->raw_lock);
> 
> Song,
> 
> take a look at s390 crash in BPF CI.
> I suspect this patch is causing it.

It indeed looks like triggered by this patch. But I haven't figured
out why it happens. v1 seems ok for the same tests. 

Song

> 
> Ilya,
> 
> do you have an idea what is going on?