linux-kernel - Re: [PATCH v12 7/8] cgroup: add a tset_get

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Message-ID: <CAOviyahr_2xwVUjhYMCSgW5om4N9N_i1VZKsoqw8S5PM-muLbg@mail.gmail.com>
Date:	Tue, 19 May 2015 11:18:41 +1000
From:	Aleksa Sarai <cyphar@...har.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	lizefan@...wei.com, mingo@...hat.com,
	Peter Zijlstra <peterz@...radead.org>, richard@....at,
	Frédéric Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org
Subject: Re: [PATCH v12 7/8] cgroup: add a tset_get_css macro

Hi Tejun,

>> +/**
>> + * tset_get_css - obtain and get css for (tset, subsys_id)
>> + * @tset: target taskset
>> + * @subsys_id: target subsystem id
>> + *
>> + * Since all of the tasks in a taskset are guaranteed to have the same css, it's
>> + * safe to grab the ref of just the first task's css and treat it as though you
>> + * have a ref on the taskset's "collective" css.
>> + */
>> +#define tset_get_css(tset, subsys_id)                                        \
>> +     task_get_css(cgroup_taskset_first(tset), subsys_id)
>
> They aren't tho.  It depends on the hierarchy configuration.  csses
> belonging to hierarchies which aren't the current migration target may
> differ.

Okay, I was confused by what you said earlier. In this case, we might
as well just move all of the attach accounting to ->attach() and not
deal with reverting it (unless we're guaranteed that the css of each
task in the tset doesn't change between ->can_attach() and
->cancel_attach()).

--
Aleksa Sarai (cyphar)
www.cyphar.com
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