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

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Date:   Fri, 27 May 2022 09:39:20 -0700
From:   Tadeusz Struk <tadeusz.struk@...aro.org>
To:     Tejun Heo <tj@...nel.org>,
        Michal Koutný <mkoutny@...e.com>
Cc:     cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Bui Quang Minh <minhquangbui99@...il.com>
Subject: Re: [PATCH 2/2] cgroup: Use separate work structs on css release path

On 5/26/22 11:15, Tejun Heo wrote:
> Hello, Michal.
> 
> On Thu, May 26, 2022 at 11:56:34AM +0200, Michal Koutný wrote:
>> // ref=A: initial state
>> kill_css()
>>    css_get // ref+=F == A+F: fuse
>>    percpu_ref_kill_and_confirm
>>      __percpu_ref_switch_to_atomic
>>        percpu_ref_get
>>          // ref += 1 == A+F+1: atomic mode, self-protection
>>      percpu_ref_put
>>        // ref -= 1 == A+F: kill the base reference
>>    [via rcu]
>>    percpu_ref_switch_to_atomic_rcu
>>      percpu_ref_call_confirm_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 -= F == A: de-fuse
>>        percpu_ref_put
>>          // ref -= 1 == A-1: remove self-protection
>>          css_release                                   // A <= 1 -> 2nd queue_work explodes!
> 
> I'm not sure I'm following it but it's perfectly fine to re-use the work
> item at this point. The work item actually can be re-cycled from the very
> beginning of the work function. The only thing we need to make sure is that
> we don't css_put() prematurely to avoid it being freed while we're using it.
> 
> For the sharing to be a problem, we should be queueing the release work item
> while the destroy instance is still pending, and if that is the case, it
> doesn't really matter whether we use two separate work items or not. We're
> already broken and would just be shifting the problem to explode elsewhere.
> 
> The only possibility that I can think of is that somehow we're ending up
> with an extra css_put() somewhere thus triggering the release path
> prematurely. If that's the case, we'll prolly need to trace get/puts to find
> out who's causing the ref imbalance.

Hi Michal,
As far as I can see we are trying to test the same thing suggested by Tejun.
I just sent a test request to try this:
https://github.com/tstruk/linux/commit/master

Let me know if you have any more tests to run and I will hold off until
you are done.

-- 
Thanks,
Tadeusz