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

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Message-ID: <51c23d0e-9f00-4433-f2e8-95113f0b2a9d@redhat.com>
Date:   Tue, 29 Nov 2022 16:13:35 -0500
From:   Waiman Long <longman@...hat.com>
To:     Hao Luo <haoluo@...gle.com>, Boqun Feng <boqun.feng@...il.com>
Cc:     Hou Tao <houtao@...weicloud.com>,
        Tonghao Zhang <xiangxia.m.yue@...il.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>
Subject: Re: [net-next] bpf: avoid hashtab deadlock with try_lock


On 11/29/22 14:36, Hao Luo wrote:
> On Tue, Nov 29, 2022 at 9:32 AM Boqun Feng <boqun.feng@...il.com> wrote:
>> Just to be clear, I meant to refactor htab_lock_bucket() into a try
>> lock pattern. Also after a second thought, the below suggestion doesn't
>> work. I think the proper way is to make htab_lock_bucket() as a
>> raw_spin_trylock_irqsave().
>>
>> Regards,
>> Boqun
>>
> The potential deadlock happens when the lock is contended from the
> same cpu. When the lock is contended from a remote cpu, we would like
> the remote cpu to spin and wait, instead of giving up immediately. As
> this gives better throughput. So replacing the current
> raw_spin_lock_irqsave() with trylock sacrifices this performance gain.
>
> I suspect the source of the problem is the 'hash' that we used in
> htab_lock_bucket(). The 'hash' is derived from the 'key', I wonder
> whether we should use a hash derived from 'bucket' rather than from
> 'key'. For example, from the memory address of the 'bucket'. Because,
> different keys may fall into the same bucket, but yield different
> hashes. If the same bucket can never have two different 'hashes' here,
> the map_locked check should behave as intended. Also because
> ->map_locked is per-cpu, execution flows from two different cpus can
> both pass.

I would suggest that you add a in_nmi() check and if true use trylock to 
get the lock. You can continue to use raw_spin_lock_irqsave() in all 
other cases.

Cheers,
Longman