linux-kernel - Re: [PATCH 1/2] mm: memcg: switch to css_tryget() in get_mem_cgroup_from

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Date:   Fri, 15 Nov 2019 19:03:03 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Tejun Heo <tj@...nel.org>
Cc:     Roman Gushchin <guro@...com>,
        Michal Koutný <mkoutny@...e.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH 1/2] mm: memcg: switch to css_tryget() in
 get_mem_cgroup_from_mm()

On Fri 15-11-19 09:48:44, Tejun Heo wrote:
> On Fri, Nov 15, 2019 at 06:47:21PM +0100, Michal Hocko wrote:
> > s@...ine@...line@
> > 
> > And reading after myself it turned out to sound differently than I
> > meant. What I wanted to say really is, what is the difference that
> > css_tryget_online really guarantee when the css might go offline right
> > after the call suceeds so more specifically what is the difference
> > between
> > 	if (css_tryget()) {
> > 		if (online)
> > 			DO_SOMETHING
> > 	}
> > and
> > 	if (css_tryget_online()) {
> > 		DO_SOMETHING
> > 	}
> > 
> > both of them are racy and do not provide any guarantee wrt. online
> > state.
> 
> It's about not giving new reference when the object is known to be
> delted to the user.

This part is clear to me. The failure just says it is already too late
to do anything. I just still struggle why the success is telling me much
more when the state might change before I can do anything on the object.
I could see a usefulness if I've had a guarantee that the object stays
online while I hold a $FOO lock but if there is nothing like that then 
we are just having already too late or potentially too late.

Anyway it's been a hard week and the brain is just going for the weekend
so I just might be dense now.

> Can you please think more about how file deletions work?

I will try that with a fresh brain next week.
-- 
Michal Hocko
SUSE Labs