linux-kernel - Re: [PATCH net-next v2] net: sched: don't disable bh when accessing action idr

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Date:   Tue, 22 May 2018 18:10:28 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Vlad Buslov <vladbu@...lanox.com>
Cc:     David Miller <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next v2] net: sched: don't disable bh when accessing
 action idr

On Mon, May 21, 2018 at 1:03 PM, Vlad Buslov <vladbu@...lanox.com> wrote:
> Initial net_device implementation used ingress_lock spinlock to synchronize
> ingress path of device. This lock was used in both process and bh context.
> In some code paths action map lock was obtained while holding ingress_lock.
> Commit e1e992e52faa ("[NET_SCHED] protect action config/dump from irqs")
> modified actions to always disable bh, while using action map lock, in
> order to prevent deadlock on ingress_lock in softirq. This lock was removed
> from net_device, so disabling bh, while accessing action map, is no longer
> necessary.
>
> Replace all action idr spinlock usage with regular calls that do not
> disable bh.

While your patch is probably fine, the above justification seems not.

In the past, tc actions could be released in BH context because tc
filters use call_rcu(). However, I moved them to a workqueue recently.
So before my change I don't think you can remove the BH protection,
otherwise race with idr_remove()...