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

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Message-ID: <20111012190754.GA10238@redhat.com>
Date:	Wed, 12 Oct 2011 21:07:54 +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
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 08:29:05PM +0200, Oleg Nesterov wrote:
> > 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.
>
> To avoid tasklist_lock in the cgroup path, or in the fork/exit path?

Sorry for confusion ;) I meant exit_notify/do_wait-like things.
I didn't mean cgroup at all.

Oleg.

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