Re: [PATCH 1/3] kfence: await for allocation using wait_event

[Marco Elver]

On Mon, 19 Apr 2021 at 11:44, Marco Elver <elver@xxxxxxxxxx> wrote:
>
> On Mon, 19 Apr 2021 at 11:41, Hillf Danton <hdanton@xxxxxxxx> wrote:
> >
> > On Mon, 19 Apr 2021 10:50:25 Marco Elver wrote:
> > > +
> > > +     WRITE_ONCE(kfence_timer_waiting, true);
> > > +     smp_mb(); /* See comment in __kfence_alloc(). */
> >
> > This is not needed given task state change in wait_event().
>
> Yes it is. We want to avoid the unconditional irq_work in
> __kfence_alloc(). When the system is under load doing frequent
> allocations, at least in my tests this avoids the irq_work almost
> always. Without the irq_work you'd be correct of course.

And in case this is about the smp_mb() here, yes it definitely is
required. We *must* order the write of kfence_timer_waiting *before*
the check of kfence_allocation_gate, which wait_event() does before
anything else (including changing the state). Otherwise the write may
be reordered after the read, and we could potentially never wake up
because __kfence_alloc() not waking us.

This is documented in __kfence_alloc().

> > > +     wait_event_timeout(allocation_wait, atomic_read(&kfence_allocation_gate), HZ);
> > > +     smp_store_release(&kfence_timer_waiting, false); /* Order after wait_event(). */
> > > +