netdev - Re: [PATCH net-next] tcp: Revert "tcp: tcp_probe: use spin_lock

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Message-ID: <CANn89iJzorQ85TTirOEsoWvVoyS-1jhahvQ873Tp38gEG-rVdA@mail.gmail.com>
Date:   Tue, 21 Feb 2017 10:46:22 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     David Miller <davem@...emloft.net>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     netdev <netdev@...r.kernel.org>,
        Ricardo Nabinger Sanchez <rnsanchez@...il.com>
Subject: Re: [PATCH net-next] tcp: Revert "tcp: tcp_probe: use spin_lock_bh()"

On Tue, Feb 21, 2017 at 10:26 AM, David Miller <davem@...emloft.net> wrote:
> From: Eric Dumazet <edumazet@...gle.com>
> Date: Tue, 21 Feb 2017 06:21:47 -0800
>
>> This reverts commit e70ac171658679ecf6bea4bbd9e9325cd6079d2b.
>>
>> jtcp_rcv_established() is in fact called with hard irq being disabled.
>>
>> Initial bug report from Ricardo Nabinger Sanchez [1] still needs
>> to be investigated, but does not look like a TCP bug.
>>
>> [1] https://www.spinics.net/lists/netdev/msg420960.html
>>
>> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
>> Reported-by: kernel test robot <xiaolong.ye@...el.com>
>> Cc: Ricardo Nabinger Sanchez <rnsanchez@...il.com>
>
> Applied.
>
> But your analysis was quite sound, we can now invoke TCP input path
> from user context, therefore this jprobe needs to use BH locking.  What
> is the problem.


If hard irq are masked, then using spin_lock() should work without a
lockdep splat.

The problem here is some false positive, as if one
trace_hardirqs_off() was missing.

Peter originally fixed the issue back in this commit :

commit 58dfe883d3bc3b4c08c53a7f39e2ca3ec84f089e
Author: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Date:   Thu Oct 11 22:25:25 2007 +0200

So really I have no idea what is happening, because the
trace_hardirqs_off() is still in x86 setjmp_pre_handler()

Maybe some FTRACE interaction ?