On Tue, 15 Dec 2020 18:15:06 -0800 Wei Wang wrote:
> On Tue, Dec 15, 2020 at 5:53 PM Jakub Kicinski <k...@kernel.org> wrote:
> > On Tue, 15 Dec 2020 17:25:14 -0800 Wei Wang wrote:
> > > +void napi_enable(struct napi_struct *n)
> > > +{
> > > + bool locked = rtnl_is_locked();
> >
> > Maybe you'll prove me wrong but I think this is never a correct
> > construct.
>
> Sorry, I don't get what you mean here.
You're checking if the mutex is locked, not if the caller is holding
the mutex.
> > > + BUG_ON(!test_bit(NAPI_STATE_SCHED, &n->state));
> > > + smp_mb__before_atomic();
> > > + clear_bit(NAPI_STATE_SCHED, &n->state);
> > > + clear_bit(NAPI_STATE_NPSVC, &n->state);
> > > + if (!locked)
> > > + rtnl_lock();
> >
> > Why do we need the lock? Can't we assume the caller of napi_enable()
> > has the sole ownership of the napi instance? Surely clearing the other
> > flags would be pretty broken as well, so the napi must had been
> > disabled when this is called by the driver.
> >
>
> Hmm... OK. The reason I added this lock operation is that we have
> ASSERT_RTNL() check in napi_set_threaded(), because it is necessary
> from the net-sysfs path to grab rtnl lock when modifying the threaded
> mode. Maybe it is not needed here.
Fair point, the sysfs write path could try to change the setting while
the NIC is starting.
> And the reason I added a rtnl_is_locked() check is I found mlx driver
> already holds rtnl lock before calling napi_enable(). Not sure about
> other drivers though.
I'd think most drivers will only mess around with napi under rtnl_lock.
> > > + WARN_ON(napi_set_threaded(n, n->dev->threaded));
> > > + if (!locked)
> > > + rtnl_unlock();
> > > +}
> > > +EXPORT_SYMBOL(napi_enable);
