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