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From:   Chengfeng Ye <dg573847474@gmail.com>
Date:   Thu, 5 Oct 2023 17:01:07 +0800
Message-ID: <CAAo+4rW=zh_d7AxJSP0uLuO7w+_PmbBfBr6D4=4X2Ays7ATqoA@mail.gmail.com>
Subject: Re: [PATCH] net/sched: use spin_lock_bh() on &gact->tcf_lock
To:     Jakub Kicinski <kuba@kernel.org>
Cc:     jhs@mojatatu.com, xiyou.wangcong@gmail.com, jiri@resnulli.us,
        davem@davemloft.net, edumazet@google.com, pabeni@redhat.com,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org
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Hi Jakub,

Thanks for the reply,

I inspected the code a bit more, it seems that the TC action is called from
tcf_proto_ops.classify() callback, which is called from Qdisc_ops enqueue
callback.

Then Qdisc enqueue callback is from

-> __dev_queue_xmit()
-> __dev_xmit_skb()
-> dev_qdisc_enqueue()

inside the net core. It seems that this __dev_queue_xmit() callback is
typically called from BH context (e.g.,  NET_TX_SOFTIRQ) with BH
already disabled, but sometimes also can from a work queue under
process context, one case is the br_mrp_test_work_expired() inside
net/bridge/br_mrp.c. Does it indicate that this TC action could also be
called with BH enable? I am not a developer so really not sure about it,
as the networking code is a bit long and complicated.

Thanks again,
Chengfeng