linux-kernel - Re: [RFC PATCH 3/3] cgroup: Do not take css_set_lock in cgroup_show

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Message-ID: <6rjdfjltz5kkwzobpeefbqxzj4wbd4jzstdryb6rb67td3x45q@5ujarspzjk3x>
Date:   Tue, 9 May 2023 12:34:53 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Dave Chinner <dchinner@...hat.com>,
        Rik van Riel <riel@...riel.com>,
        Jiri Wiesner <jwiesner@...e.de>
Subject: Re: [RFC PATCH 3/3] cgroup: Do not take css_set_lock in
 cgroup_show_path

On Fri, May 05, 2023 at 08:17:10AM -1000, Tejun Heo <tj@...nel.org> wrote:
> On Fri, May 05, 2023 at 07:32:40PM +0200, Michal Koutný wrote:
> > On Fri, May 05, 2023 at 05:45:58AM -1000, Tejun Heo <tj@...nel.org> wrote:
> > > > There are three relevant nodes for each cgroupfs entry:
> > > > 
> > > >         R ... cgroup hierarchy root
> > > >         M ... mount root
> > > >         C ... reader's cgroup NS root
> > > > 
> > > > mountinfo is supposed to show path from C to M.
> > > 
> > > At least for cgroup2, the path from C to M isn't gonna change once NS is
> > > established, right?
> > 
> > Right. Although, the argument about M (when C above M or when C and M in
> > different subtrees) implicitly relies on the namespace_sem.
> 
> I don't follow. Can you please elaborate a bit more?

I wanted to say that even with restriction to cgroup2, the css_set_lock
removal would also rely on namespace_sem.

For a given mountinfo entry the path C--M won't change (no renames).
The question is whether cgroup M will stay around (with the relaxed
locking):

  - C >= M (C is below M) 
    -> C (transitively) pins M

  - C < M (C is above M) or C and M are in two disjoint subtrees (path
    goes through a common ancestor)
    -> M could be released without relation to C (even on cgroup2, with
       the css_set_lock removed) but such a destructive operation on M
       is excluded as long as namespace_sem is held during entry
       rendering.

Does that clarify the trade-off of removing css_set_lock at this spot?

Thanks,
Michal

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