Re: Lockdep warning after c0feea594e058223973db94c1c32a830c9807c86

Jakub Sitnicki
2022-10-13T20:52:19Z

Hi Stan,

On Wed, Oct 12, 2022 at 02:08 PM -07, sdf@google.com wrote:
> Hi John & Jakub,
>
> Upstream commit c0feea594e05 ("workqueue: don't skip lockdep work
> dependency in cancel_work_sync()") seems to trigger the following
> lockdep warning during test_prog's sockmap_listen:
>
> [ +0.003631] WARNING: possible circular locking dependency detected

[...]

> Are you ware? Any idea what's wrong?
> Is there some stable fix I'm missing in bpf-next?

Thanks for bringing it up. I didn't know.

The mentioned commit doesn't look that fresh

commit c0feea594e058223973db94c1c32a830c9807c86
Author: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Date: Fri Jul 29 13:30:23 2022 +0900

workqueue: don't skip lockdep work dependency in cancel_work_sync()

... but then it just landed not so long ago, which explains things:

$ git describe --contains c0feea594e058223973db94c1c32a830c9807c86 --match 'v*'
v6.0-rc7~10^2

I've untangled the call chains leading to the potential dead-lock a
bit. There does seem to be a window of opportunity there.

psock->work.func = sk_psock_backlog()
ACQUIRE psock->work_mutex
sk_psock_handle_skb()
skb_send_sock()
__skb_send_sock()
sendpage_unlocked()
kernel_sendpage()
sock->ops->sendpage = inet_sendpage()
sk->sk_prot->sendpage = tcp_sendpage()
ACQUIRE sk->sk_lock
tcp_sendpage_locked()
RELEASE sk->sk_lock
RELEASE psock->work_mutex

sock_map_close()
ACQUIRE sk->sk_lock
sk_psock_stop()
sk_psock_clear_state(psock, SK_PSOCK_TX_ENABLED)
cancel_work_sync()
__cancel_work_timer()
__flush_work()
// wait for psock->work to finish
RELEASE sk->sk_lock

There is no fix I know of. Need to think. Ideas welcome.

CC Cong, just FYI, because we did rearrange the locking scheme in [1].

However it looks to me like the dead-lock was already there before that.

[1] https://lore.kernel.org/bpf/20210331023237.41094-5-xiyou.wangcong@gmail.com/