netdev - Re: [Patch net-next] net_sched: call qdisc

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Date:   Tue, 02 Jan 2018 11:39:13 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     xiyou.wangcong@...il.com
Cc:     netdev@...r.kernel.org, jakub.kicinski@...ronome.com,
        john.fastabend@...il.com
Subject: Re: [Patch net-next] net_sched: call qdisc_reset() with qdisc lock

From: Cong Wang <xiyou.wangcong@...il.com>
Date: Thu, 21 Dec 2017 16:03:29 -0800

> qdisc_reset() should always be called with qdisc spinlock
> and with BH disabled, otherwise qdisc ->reset() could race
> with TX BH.
> 
> Fixes: 7bbde83b1860 ("net: sched: drop qdisc_reset from dev_graft_qdisc")
> Reported-by: Jakub Kicinski <jakub.kicinski@...ronome.com>
> Cc: John Fastabend <john.fastabend@...il.com>
> Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>

There doesn't seem to be agreement over whether qdisc_reset() really
needs to run with the lock held.  In fact, the general consensus is
that this really shouldn't run until a grace period had occurred and
therefore parallel TX paths cannot be running any longer.

In any event, this was supposed to work towards a bug fix which
ultimately was fixed instead with the ptr_ring change from John.

So this seems unnecessary now.

If you disagree, please repost with an updated commit message which
explains things further.

Thank you.