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

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Message-ID: <20191115174844.GR4163745@devbig004.ftw2.facebook.com>
Date:   Fri, 15 Nov 2019 09:48:44 -0800
From:   Tejun Heo <tj@...nel.org>
To:     Michal Hocko <mhocko@...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, 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.  Can you please think more about how file
deletions work?

Thanks.

-- 
tejun