Subject: Re: [PATCH 1/4] add task handling notifier: base definitions
From: Matt Helsley <matthltc@us.ibm.com>
To: Jan Beulich <jbeulich@novell.com>
Cc: Paul Jackson <pj@sgi.com>, linux-kernel@vger.kernel.org
In-Reply-To: <4784A601.76E4.0078.0@novell.com>
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Organization: IBM Linux Technology Center
Date: Wed, 23 Jan 2008 11:11:25 -0800
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On Wed, 2008-01-09 at 09:46 +0000, Jan Beulich wrote:
> >> +BLOCKING_NOTIFIER_HEAD(task_notifier_list);
> >> +EXPORT_SYMBOL_GPL(task_notifier_list);
> >> +ATOMIC_NOTIFIER_HEAD(atomic_task_notifier_list);
> >> +EXPORT_SYMBOL_GPL(atomic_task_notifier_list);
> >> +
> >
> >When these global notifier lists were proposed years ago folks at SGI
> >loudly objected with concerns over anticipated cache line bouncing on
> >512+ cpu machines. Is that no longer a concern?
> 
> I can't see an alternative, since the serialization is unavoidable.

There are definitely alternatives. Naturally they are all much more
complex than using a single notifier chain. You could do per-cpu chains
for example (ugly, I know..).

> >> @@ -121,6 +127,9 @@ void __put_task_struct(struct task_struc
> >>  	WARN_ON(atomic_read(&tsk->usage));
> >>  	WARN_ON(tsk == current);
> >> 
> >> +	atomic_notifier_call_chain(&atomic_task_notifier_list,
> >> +				   TASK_DELETE, tsk);
> >> +
> >>  	security_task_free(tsk);
> >>  	free_uid(tsk->user);
> >>  	put_group_info(tsk->group_info);
> >
> >Would the atomic notifier call chain be necessary if you hooked into an
> >earlier section of do_exit() instead?
> 
> I'm afraid it is, as I was told that sleeping in the do_exit() path is not
> generally possible.
> 
> Jan

Odd. Last I checked I thought I saw a bunch of calls in do_exit() that
could sleep. Only in certain sections and at the end did it appear to
prevent sleeping.

Sorry for the late reply.

Cheers,
	-Matt Helsley

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