netdev - Re: [Patch net-next v2] mlx5: use RCU lock in mlx5_eq_cq

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Date:   Wed, 6 Feb 2019 16:56:25 -0800
From:   Saeed Mahameed <saeedm@....mellanox.co.il>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Tariq Toukan <tariqt@...lanox.com>
Subject: Re: [Patch net-next v2] mlx5: use RCU lock in mlx5_eq_cq_get()

On Wed, Feb 6, 2019 at 4:53 PM Cong Wang <xiyou.wangcong@...il.com> wrote:
>
> On Wed, Feb 6, 2019 at 4:28 PM Eric Dumazet <eric.dumazet@...il.com> wrote:
> >
> >
> >
> > On 02/06/2019 04:04 PM, Cong Wang wrote:
> >
> > > synchronize_irq() is called before mlx5_cq_put(), so I don't
> > > see why readers could get 0 refcnt.
> >
> > Then the more reasons to get rid of the refcount increment/decrement completely ...
> >
> > Technically, even the rcu_read_lock() and rcu_read_unlock() are not needed,
> > since synchronize_irq() is enough.
>
> Excellent point.
>
> For the refcnt, I am afraid we still have to hold refcnt for the tasklet,
> mlx5_cq_tasklet_cb. But yeah, should be safe to remove from IRQ
> path.

the tasklet path is for rdma CQs only, netdev cqs handling will be refcnt free.