linux-kernel - Re: [PATCH v4] sched: automated per session task groups

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Message-ID: <20101121154435.GA25966@redhat.com>
Date:	Sun, 21 Nov 2010 16:44:35 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Mike Galbraith <efault@....de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4] sched: automated per session task groups

On 11/21, Ingo Molnar wrote:
>
> Btw., there's a small cleanup in the patch that i picked up (see below), and i also
> edited the commit log a bit - so you might want to pick up the version below.

I didn't read the patch in details, but a couple of nits...

> +static void
> +autogroup_move_group(struct task_struct *p, struct autogroup *ag)
> +{
> +	struct autogroup *prev;
> +	struct task_struct *t;
> +
> +	spin_lock(&p->sighand->siglock);

This needs spin_lock_irq(), ->siglock is irq-safe.

The same for other lockers, but:

> +static inline struct autogroup *autogroup_get(struct task_struct *p)
> +{
> +	struct autogroup *ag;
> +
> +	/* task may be moved after we unlock.. tough */
> +	spin_lock(&p->sighand->siglock);

This is called by fs/proc. In this case nothing protects us from
release_task(), we can hit ->siglock == NULL (or we can race with
exec which changes ->sighand in theory).

This needs lock_task_sighand() (it can fail). Perhaps something
else have the same problem...

If the task is current and it is not exiting, or it is the new
child (sched_autogroup_fork), then it is safe to use ->siglock
directly.

And a pure cosmetic nit,

> +void sched_autogroup_fork(struct signal_struct *sig)
> +{
> +     struct sighand_struct *sighand = current->sighand;
> +
> +     spin_lock(&sighand->siglock);
> +     sig->autogroup = autogroup_kref_get(current->signal->autogroup);
> +     spin_unlock(&sighand->siglock);
> +}

This looks a bit confusing. We do not need current->sighand->siglock
to set sig->autogroup. Nobody except us can see this new signal_struct,
and in any case current->sighand->siglock can't help.

It is needed for autogroup_kref_get(), but we already have autogroup_get().
I'd suggest

	void sched_autogroup_fork(struct signal_struct *sig)
	{
		sig->autogroup = autogroup_get(current);	
	}

Oleg.

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