Re: BPF: unbounded bpf_map_free_deferred problem

From: Cong Wang <xiyou.wangcong@gmail.com>
To: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: netdev <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Tejun Heo <tj@kernel.org>, Alexey Kardashevskiy <aik@ozlabs.ru>,
	Dmitry Vyukov <dvyukov@google.com>
Subject: Re: BPF: unbounded bpf_map_free_deferred problem
Date: Fri, 22 Jan 2021 19:27:35 -0800	[thread overview]
Message-ID: <CAM_iQpU3P03+2QL2iDbVQSyqwHb6DXi96eXNEm3kDgFWjqAKHg@mail.gmail.com> (raw)
In-Reply-To: <b9e41542-5c93-9d37-d99d-acde6fb01fa1@i-love.sakura.ne.jp>

On Fri, Jan 22, 2021 at 4:42 PM Tetsuo Handa
<penguin-kernel@i-love.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.