linux-kernel - Re: [PATCH 2/2] cgroup: Use separate work structs on css release path

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Message-ID: <20220525161455.GA16134@blackbody.suse.cz>
Date:   Wed, 25 May 2022 18:14:55 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Bui Quang Minh <minhquangbui99@...il.com>,
        Tadeusz Struk <tadeusz.struk@...aro.org>
Subject: Re: [PATCH 2/2] cgroup: Use separate work structs on css release path

On Wed, May 25, 2022 at 05:15:17PM +0200, Michal Koutný <mkoutny@...e.com> wrote:
> // ref=1: only base reference
> kill_css()
>   css_get() // fuse, ref+=1 == 2
>   percpu_ref_kill_and_confirm
>     // ref -= 1 == 1: kill base references
>   [via rcu]
>   css_killed_ref_fn == refcnt.confirm_switch
>     queue_work(css->destroy_work)               (1)
>     [via css->destroy_work]
>     css_killed_work_fn == wq.func
>       offline_css() // needs fuse
>       css_put // ref -= 1 == 0: de-fuse, was last
>         ...
>         percpu_ref_put_many
>            css_release
>              queue_work(css->destroy_work)      (2)
>              [via css->destroy_work]
>              css_release_work_fn == wq.func

Apologies, this is wrong explanation. (I thought this explains why
Tadeusz's patch with double get/put didn't fix it (i.e. any number
wouldn't help with the explanation above).)

But the above is not correct. I've looked at the stack trace [1] and the
offending percpu_ref_put_many is called from an RCU callback
percpu_ref_switch_to_atomic_rcu(), so I can't actually see why it drops
to zero there...

Regards,
Michal