linux-kernel - Re: BPF: unbounded bpf_map_free

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Date:   Fri, 22 Jan 2021 19:27:35 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Tejun Heo <tj@...nel.org>,
        Alexey Kardashevskiy <aik@...abs.ru>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: BPF: unbounded bpf_map_free_deferred problem

On Fri, Jan 22, 2021 at 4:42 PM Tetsuo Handa
<penguin-kernel@...ove.sakura.ne.jp> wrote:
>
> Hello, BPF developers.
>
> Alexey Kardashevskiy is reporting that system_wq gets stuck due to flooding of
> unbounded bpf_map_free_deferred work. Use of WQ_MEM_RECLAIM | WQ_HIGHPRI | WQ_UNBOUND
> workqueue did not solve this problem. Is it possible that a refcount leak somewhere
> preventing bpf_map_free_deferred from completing? Please see
> https://lkml.kernel.org/r/CACT4Y+Z+kwPM=WUzJ-e359PWeLLqmF0w4Yxp1spzZ=+J0ekrag@mail.gmail.com .
>

Which map does the reproducer create? And where exactly do
those work block on?

Different map->ops->map_free() waits for different reasons,
for example, htab_map_free() waits for flying htab_elem_free_rcu().
I can't immediately see how they could wait for each other, if this
is what you meant above.

Thanks.