netdev - Re: [PATCH v2 net-next 9/9] l2tp: flush workqueue before draining it

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Message-ID: <Zrj6w89B7so74jRU@pop-os.localdomain>
Date: Sun, 11 Aug 2024 10:54:11 -0700
From: Cong Wang <xiyou.wangcong@...il.com>
To: James Chapman <jchapman@...alix.com>
Cc: netdev@...r.kernel.org, davem@...emloft.net, edumazet@...gle.com,
	kuba@...nel.org, pabeni@...hat.com, dsahern@...nel.org,
	tparkin@...alix.com, horms@...nel.org,
	syzbot+0e85b10481d2f5478053@...kaller.appspotmail.com
Subject: Re: [PATCH v2 net-next 9/9] l2tp: flush workqueue before draining it

On Wed, Aug 07, 2024 at 07:54:52AM +0100, James Chapman wrote:
> syzbot exposes a race where a net used by l2tp is removed while an
> existing pppol2tp socket is closed. In l2tp_pre_exit_net, l2tp queues
> TUNNEL_DELETE work items to close each tunnel in the net. When these
> are run, new SESSION_DELETE work items are queued to delete each
> session in the tunnel. This all happens in drain_workqueue. However,
> drain_workqueue allows only new work items if they are queued by other
> work items which are already in the queue. If pppol2tp_release runs
> after drain_workqueue has started, it may queue a SESSION_DELETE work
> item, which results in the warning below in drain_workqueue.
> 
> Address this by flushing the workqueue before drain_workqueue such
> that all queued TUNNEL_DELETE work items run before drain_workqueue is
> started. This will queue SESSION_DELETE work items for each session in
> the tunnel, hence pppol2tp_release or other API requests won't queue
> SESSION_DELETE requests once drain_workqueue is started.
> 

I am not convinced here.

1) There is a __flush_workqueue() inside drain_workqueue() too. Why
calling it again?

2) What prevents new work items be queued right between your
__flush_workqueue() and drain_workqueue()?

Thanks.