> On Oct 3, 2023, at 8:33 PM, Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Tue, Oct 3, 2023 at 8:08 PM Andrii Nakryiko
> <andrii.nakryiko@xxxxxxxxx> wrote:
>>
>> On Tue, Oct 3, 2023 at 5:45 PM Song Liu <song@xxxxxxxxxx> 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@xxxxxxxxxx>
>>> Signed-off-by: Song Liu <song@xxxxxxxxxx>
>>>
>>> ---
>>> 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@xxxxxxxxxx>
>>
>>> 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?
