Re: [PATCH 1/2] mm: memcg: switch to css_tryget() in get_mem_cgroup_from_mm()

From: Roman Gushchin <guro@fb.com>
To: Michal Hocko <mhocko@kernel.org>
Cc: "Tejun Heo" <tj@kernel.org>, "Michal Koutný" <mkoutny@suse.com>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Johannes Weiner" <hannes@cmpxchg.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"Kernel Team" <Kernel-team@fb.com>,
	"stable@vger.kernel.org" <stable@vger.kernel.org>
Subject: Re: [PATCH 1/2] mm: memcg: switch to css_tryget() in get_mem_cgroup_from_mm()
Date: Fri, 15 Nov 2019 18:07:34 +0000	[thread overview]
Message-ID: <20191115180728.GA27385@localhost.localdomain> (raw)
In-Reply-To: <20191115174721.GB15216@dhcp22.suse.cz>

On Fri, Nov 15, 2019 at 06:47:21PM +0100, Michal Hocko wrote:
> On Fri 15-11-19 18:40:31, Michal Hocko wrote:
> > On Thu 14-11-19 11:37:36, Tejun Heo wrote:
> > > Hello,
> > > 
> > > On Thu, Nov 14, 2019 at 08:33:40PM +0100, Michal Hocko wrote:
> > > > > It is useful for controlling admissions of new userspace visible uses
> > > > > - e.g. a tracepoint shouldn't be allowed to be attached to a cgroup
> > > > > which has already been deleted.
> > > > 
> > > > I am not sure I understand. Roman says that the cgroup can get offline
> > > > right after the function returns. How is "already deleted" different
> > > > from "just deleted"? I thought that the state is preserved at least
> > > > while the rcu lock is held but my memory is dim here.
> > > 
> > > It's the same difference as between "opening a file and deleting it"
> > > and "deleting a file and opening it".
> > 
> > I am sorry but I do not follow. How can css_tryget_online provide the
> > same semantic when the css can go offline right after the tryget call
> > returns so it is effectivelly undistinguishable from the case when the
> > css was already online before the call was made.
> 
> s@online@offline@
> 
> 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.

Let me step back a little bit.

I think, we all agree that css_tryget_online() has a weird semantics,
in most cases is used only due to historical reasons and clearly asks
for a cleanup. So I suggest to stop arguing about it and wait for the
cleanup patchset. Then we can discuss each remaining use case in details,
if there will be any.

Thanks!