linux-kernel - Re: [net-next] bpf: avoid hashtab deadlock with try

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Message-ID: <CA+khW7hsvueaRRFX3m-gxtW0A7fYEOJLfDTSTVMY-OLn_si1hQ@mail.gmail.com>
Date:   Tue, 29 Nov 2022 21:02:11 -0800
From:   Hao Luo <haoluo@...gle.com>
To:     Hou Tao <houtao@...weicloud.com>
Cc:     Tonghao Zhang <xiangxia.m.yue@...il.com>,
        Waiman Long <longman@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        netdev@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Jiri Olsa <jolsa@...nel.org>, bpf <bpf@...r.kernel.org>,
        "houtao1@...wei.com" <houtao1@...wei.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Boqun Feng <boqun.feng@...il.com>
Subject: Re: [net-next] bpf: avoid hashtab deadlock with try_lock

On Tue, Nov 29, 2022 at 8:13 PM Hou Tao <houtao@...weicloud.com> wrote:
>
> On 11/30/2022 10:47 AM, Tonghao Zhang wrote:
<...>
> >         if (in_nmi()) {
> >                 if (!raw_spin_trylock_irqsave(&b->raw_lock, flags))
> >                         return -EBUSY;
>
> The only purpose of trylock here is to make lockdep happy and it may lead to
> unnecessary -EBUSY error for htab operations in NMI context. I still prefer add
> a virtual lock-class for map_locked to fix the lockdep warning. So could you use
> separated patches to fix the potential dead-lock and the lockdep warning ? It
> will be better you can also add a bpf selftests for deadlock problem as said before.
>

Agree with Tao here. Tonghao, could you send another version which:

- separates the fix to deadlock and the fix to lockdep warning
- includes a bpf selftest to verify the fix to deadlock
- with bpf-specific tag: [PATCH bpf-next]

There are multiple ideas on the fly in this thread, it's easy to lose
track of what has been proposed and what change you intend to make.

Thanks,
Hao