netdev - Re: [PATCH net-next v5 2/3] net: implement threaded-able napi poll loop support

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Message-ID: <CAEA6p_DdarvYYSdnof+mEWJ=R7gBLB0By9EME2ZEfvGBhhN+Yg@mail.gmail.com>
Date:   Tue, 15 Dec 2020 18:15:06 -0800
From:   Wei Wang <weiwan@...gle.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     David Miller <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Felix Fietkau <nbd@....name>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net-next v5 2/3] net: implement threaded-able napi poll
 loop support

On Tue, Dec 15, 2020 at 5:53 PM Jakub Kicinski <kuba@...nel.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.

>
> > +     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.
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.

> > +     WARN_ON(napi_set_threaded(n, n->dev->threaded));
> > +     if (!locked)
> > +             rtnl_unlock();
> > +}
> > +EXPORT_SYMBOL(napi_enable);
>
> Let's switch to RFC postings and get it in for 5.12 :(

OK.