netdev - Re: [PATCH net-next 2/2] pfifo_fast: drop unneeded additional lock on dequeue

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Date:   Wed, 16 May 2018 11:57:23 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        John Fastabend <john.fastabend@...il.com>
Subject: Re: [PATCH net-next 2/2] pfifo_fast: drop unneeded additional lock
 on dequeue

On Wed, 2018-05-16 at 09:56 +0200, Paolo Abeni wrote:
> On Tue, 2018-05-15 at 23:17 +0300, Michael S. Tsirkin wrote:
> > On Tue, May 15, 2018 at 04:24:37PM +0200, Paolo Abeni wrote:
> > > After the previous patch, for NOLOCK qdiscs, q->seqlock is
> > > always held when the dequeue() is invoked, we can drop
> > > any additional locking to protect such operation.
> > > 
> > > Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> > > ---
> > >  include/linux/skb_array.h | 5 +++++
> > >  net/sched/sch_generic.c   | 4 ++--
> > >  2 files changed, 7 insertions(+), 2 deletions(-)
> > 
> > Is the seqlock taken during qdisc_change_tx_queue_len?
> > We need to prevent that racing with dequeue.
> 
> Thanks for the head-up! I missed that code-path.
> 
> I'll add the lock in qdisc_change_tx_queue_len() in v2.

Actually the lock is not needed in qdisc_change_tx_queue_len(): the
device is deactivated before calling ops->change_tx_queue_len, so the
latter can't race with ops->dequeue().

I think the current patch is safe.

Cheers,

Paolo