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From: "Michal Koutný" <mkoutny@suse.com>
To: Yafang Shao <laoar.shao@gmail.com>
Cc: ast@kernel.org, daniel@iogearbox.net, john.fastabend@gmail.com,
	 andrii@kernel.org, martin.lau@linux.dev, song@kernel.org,
	yonghong.song@linux.dev,  kpsingh@kernel.org, sdf@google.com,
	haoluo@google.com, jolsa@kernel.org,  tj@kernel.org,
	lizefan.x@bytedance.com, hannes@cmpxchg.org,
	 yosryahmed@google.com, sinquersw@gmail.com,
	cgroups@vger.kernel.org,  bpf@vger.kernel.org
Subject: Re: [RFC PATCH bpf-next 1/8] cgroup: Don't have to hold cgroup_mutex in task_cgroup_from_root()
Date: Tue, 10 Oct 2023 10:29:52 +0200	[thread overview]
Message-ID: <afdnpo3jz2ic2ampud7swd6so5carkilts2mkygcaw67vbw6yh@5b5mncf7qyet> (raw)
In-Reply-To: <CALOAHbC4_0990_HD4=mg8gfU51juk8fs07zYrr6VL9fPOuLOng@mail.gmail.com>

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Hi Yafang.

On Tue, Oct 10, 2023 at 11:58:14AM +0800, Yafang Shao <laoar.shao@gmail.com> wrote:
> current_cgns_cgroup_from_root() doesn't hold the cgroup_mutext as
> well. Could this potentially lead to issues, such as triggering the
> BUG_ON() in __cset_cgroup_from_root(), if the root has already been
> destroyed?

current_cgns_cgroup_from_root() is a tricky one, see also
https://lore.kernel.org/r/20230502133847.14570-3-mkoutny@suse.com/

I argued there with RCU read lock but when I look at it now, it may not be
sufficient for the cgroup returned from current_cgns_cgroup_from_root().

The 2nd half still applies, umount synchronization is ensured via VFS
layer, so the cgroup_root nor its cgroup won't go away in the
only caller cgroup_show_path().


> Would it be beneficial to introduce a dedicated root_list_lock
> specifically for this purpose? This approach could potentially reduce
> the need for the broader cgroup_mutex in other scenarios.

It may be a convenience lock but v2 (cgrp_dfl_root could make do just
fine without it).

I'm keeping this dicussuion to illustrate the difficulties of adding the
BPF support for cgroup v1. That is a benefit I see ;-)

Michal


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  reply	other threads:[~2023-10-10  8:30 UTC|newest]

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-10-07 14:02 [RFC PATCH bpf-next 0/8] bpf, cgroup: Add BPF support for cgroup1 hierarchy Yafang Shao
2023-10-07 14:02 ` [RFC PATCH bpf-next 1/8] cgroup: Don't have to hold cgroup_mutex in task_cgroup_from_root() Yafang Shao
2023-10-07 15:50   ` Tejun Heo
2023-10-08  2:32     ` Yafang Shao
2023-10-09 14:45   ` Michal Koutný
2023-10-10  3:58     ` Yafang Shao
2023-10-10  8:29       ` Michal Koutný [this message]
2023-10-10 11:58         ` Yafang Shao
2023-10-07 14:02 ` [RFC PATCH bpf-next 2/8] cgroup: Add new helpers for cgroup1 hierarchy Yafang Shao
2023-10-07 15:55   ` Tejun Heo
2023-10-08  2:36     ` Yafang Shao
2023-10-09 11:32   ` Michal Koutný
2023-10-09 13:10     ` Yafang Shao
2023-10-09 14:48       ` Michal Koutný
2023-10-10  3:59         ` Yafang Shao
2023-10-07 14:02 ` [RFC PATCH bpf-next 3/8] bpf: Add kfuncs " Yafang Shao
2023-10-07 15:57   ` Tejun Heo
2023-10-08  2:37     ` Yafang Shao
2023-10-07 14:03 ` [RFC PATCH bpf-next 4/8] selftests/bpf: Fix issues in setup_classid_environment() Yafang Shao
2023-10-07 14:03 ` [RFC PATCH bpf-next 5/8] selftests/bpf: Add parallel support for classid Yafang Shao
2023-10-07 14:03 ` [RFC PATCH bpf-next 6/8] selftests/bpf: Add a new cgroup helper get_classid_cgroup_id() Yafang Shao
2023-10-07 14:03 ` [RFC PATCH bpf-next 7/8] selftests/bpf: Add a new cgroup helper get_cgroup_hierarchy_id() Yafang Shao
2023-10-07 14:03 ` [RFC PATCH bpf-next 8/8] selftests/bpf: Add selftests for cgroup1 hierarchy Yafang Shao
2023-10-09 11:46 ` [RFC PATCH bpf-next 0/8] bpf, cgroup: Add BPF support " Michal Koutný
2023-10-09 13:11   ` Yafang Shao

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