netdev - Re: [Patch bpf] sock_map: convert cancel_work_sync() to cancel

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Date:   Mon, 31 Oct 2022 23:03:09 +0100
From:   Jakub Sitnicki <jakub@...udflare.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Cong Wang <cong.wang@...edance.com>, sdf@...gle.com,
        netdev@...r.kernel.org, bpf@...r.kernel.org,
        John Fastabend <john.fastabend@...il.com>
Subject: Re: [Patch bpf] sock_map: convert cancel_work_sync() to cancel_work()

On Fri, Oct 28, 2022 at 12:16 PM -07, Cong Wang wrote:
> On Mon, Oct 24, 2022 at 03:33:13PM +0200, Jakub Sitnicki wrote:
>> On Tue, Oct 18, 2022 at 11:13 AM -07, sdf@...gle.com wrote:
>> > On 10/17, Cong Wang wrote:
>> >> From: Cong Wang <cong.wang@...edance.com>
>> >
>> >> Technically we don't need lock the sock in the psock work, but we
>> >> need to prevent this work running in parallel with sock_map_close().
>> >
>> >> With this, we no longer need to wait for the psock->work synchronously,
>> >> because when we reach here, either this work is still pending, or
>> >> blocking on the lock_sock(), or it is completed. We only need to cancel
>> >> the first case asynchronously, and we need to bail out the second case
>> >> quickly by checking SK_PSOCK_TX_ENABLED bit.
>> >
>> >> Fixes: 799aa7f98d53 ("skmsg: Avoid lock_sock() in sk_psock_backlog()")
>> >> Reported-by: Stanislav Fomichev <sdf@...gle.com>
>> >> Cc: John Fastabend <john.fastabend@...il.com>
>> >> Cc: Jakub Sitnicki <jakub@...udflare.com>
>> >> Signed-off-by: Cong Wang <cong.wang@...edance.com>
>> >
>> > This seems to remove the splat for me:
>> >
>> > Tested-by: Stanislav Fomichev <sdf@...gle.com>
>> >
>> > The patch looks good, but I'll leave the review to Jakub/John.
>> 
>> I can't poke any holes in it either.
>> 
>> However, it is harder for me to follow than the initial idea [1].
>> So I'm wondering if there was anything wrong with it?
>
> It caused a warning in sk_stream_kill_queues() when I actually tested
> it (after posting).

We must have seen the same warnings. They seemed unrelated so I went
digging. We have a fix for these [1]. They were present since 5.18-rc1.

>> This seems like a step back when comes to simplifying locking in
>> sk_psock_backlog() that was done in 799aa7f98d53.
>
> Kinda, but it is still true that this sock lock is not for sk_socket
> (merely for closing this race condition).

I really think the initial idea [2] is much nicer. I can turn it into a
patch, if you are short on time.

With [1] and [2] applied, the dead lock and memory accounting warnings
are gone, when running `test_sockmap`.

Thanks,
Jakub

[1] https://lore.kernel.org/netdev/1667000674-13237-1-git-send-email-wangyufen@huawei.com/
[2] https://lore.kernel.org/netdev/Y0xJUc%2FLRu8K%2FAf8@pop-os.localdomain/