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

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Date:   Tue, 29 Nov 2022 11:36:23 -0800
From:   Hao Luo <haoluo@...gle.com>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     Waiman Long <longman@...hat.com>, 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 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.

Hao