linux-kernel - Re: [PATCH 3/4] threadgroup: extend threadgroup

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Message-ID: <20111012182905.GA8580@redhat.com>
Date:	Wed, 12 Oct 2011 20:29:05 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Ben Blum <bblum@...rew.cmu.edu>
Cc:	Tejun Heo <htejun@...il.com>, rjw@...k.pl, paul@...lmenage.org,
	lizf@...fujitsu.com, linux-pm@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org,
	containers@...ts.linux-foundation.org, fweisbec@...il.com,
	matthltc@...ibm.com, akpm@...ux-foundation.org,
	Paul Menage <menage@...gle.com>
Subject: Re: [PATCH 3/4] threadgroup: extend threadgroup_lock() to cover
	exit and exec

On 10/12, Ben Blum wrote:
>
> On Wed, Oct 12, 2011 at 07:51:04PM +0200, Oleg Nesterov wrote:
> >
> > > Also, it makes the mechanism unnecessarily cgroup-specific without
> > > gaining much if anything.
> >
> > Yes! And _personally_ I think it should be cgroup-specific, that is
> > why I dislike the very fact do_exit() uses it directly. To me it would
> > be cleaner to shift it into cgroup hooks. Yes, sure, this is subjective.
>
> In the fork path, threadgroup_fork_read_...() is also called directly,
> not through cgroups. Would that change too?

Well, if you ask me, I'd prefer to move lock/unlock into
cgroup_fork/cgroup_post_fork ;) Although the error path plays with it
too. But this is minor.

> > In fact I still hope we can kill this sem altogether, but so far I have
> > no idea how we can do this. We do need the new per-process lock to
> > protect (in particular) ->thread_group. It is quite possible that it
> > should be rw_semaphore. But in this case we down_write(), not _read
> > in exit/fork paths, and its scope should be small.
>
> I'm confused - taking a big rwsem for writing in the fork/exit paths?

Yes, we need the new lock to avoid tasklist_lock.

> The point here is that even though fork/exit modify thread_group, they
> are logical "readers"

Yes I see. And this is one of the reasons why we can't use this lock
for above.

Oleg.

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