linux-kernel - Re: [PATCH net-next v15 03/14] netdev: support binding dma-buf to netdevice

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Message-ID: <CAHS8izOCuNZWfZR_jecFOMu2XGqcYUkuVf38wRqBvoE9tmGzoQ@mail.gmail.com>
Date: Wed, 3 Jul 2024 09:56:45 -0700
From: Mina Almasry <almasrymina@...gle.com>
To: Jakub Kicinski <kuba@...nel.org>
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Subject: Re: [PATCH net-next v15 03/14] netdev: support binding dma-buf to netdevice

On Tue, Jul 2, 2024 at 6:09 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Fri, 28 Jun 2024 00:32:40 +0000 Mina Almasry wrote:
> > +     if (binding->list.next)
> > +             list_del(&binding->list);
> > +
> > +     xa_for_each(&binding->bound_rxq_list, xa_idx, rxq) {
>
> nit: s/bound_rxq_list/bound_rxqs/ ? it's not a list
>
> > +             if (rxq->mp_params.mp_priv == binding) {
> > +                     /* We hold the rtnl_lock while binding/unbinding
> > +                      * dma-buf, so we can't race with another thread that
> > +                      * is also modifying this value. However, the page_pool
> > +                      * may read this config while it's creating its
> > +                      * rx-queues. WRITE_ONCE() here to match the
> > +                      * READ_ONCE() in the page_pool.
> > +                      */
> > +                     WRITE_ONCE(rxq->mp_params.mp_priv, NULL);
>
> Is this really sufficient in terms of locking? @binding is not
> RCU-protected and neither is the reader guaranteed to be in
> an RCU critical section. Actually the "reader" tries to take a ref
> and use this struct so it's not even a pure reader.
>
> Let's add a lock or use one of the existing locks
>

Can we just use rtnl_lock() for this synchronization? It seems it's
already locked everywhere that access mp_params.mp_priv, so the
WRITE/READ_ONCE are actually superfluous. Both the dmabuf bind/unbind
already lock rtnl_lock, and the only other place that access
mp_params.mp_priv is page_pool_init(). I think it's reasonable to
assume rtnl_lock is also held during page_pool_init, no? AFAICT it
would be very weird for some code path to be reconfiguring the driver
page_pools without holding rtnl_lock?

What I wanna do here is delete the incorrect comment, remove the
READ/WRITE_ONCE, and maybe add a DEBUG_NET_WARN_ON(!rtnl_is_locked())
in mp_dmabuf_devmem_init() but probably that is too defensive.

Will apply the other comments, thanks.

-- 
Thanks,
Mina