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Date: Sun, 28 Mar 2010 04:07:10 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
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Subject: Re: [patch 0/6] rcu head debugobjects
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B1;2005;0cOn Sat, 27 Mar 2010, Paul E. McKenney wrote:
> 
> o	Patch 4/6 looks good to me, and given that Thomas hasn't
> 	complained, I am guessing that he is OK with it.

Looks sane at the first glance. Will go over it in detail tomorrow.
 
> o	Patch 6/6: Would it be possible to use the object_is_on_stack()
> 	function defined in include/linux/sched.h instead of passing
> 	in the flag on_stack to bdi_work_init()?  It looks like
> 	fs/fs-writeback.c already includes include/linux/sched.h, so
> 	shouldn't be a problem from a #include-hell viewpoint.

Well, I'm a bit wary about that. The reason is that we really want
the annotation of:

       init_on_stack();
       ....
       destroy_on_stack();

instead of the confusing:

       init();
       ....
       destroy_on_stack();

So having an automatism in the bdi_work_init() function will people
make forget to put the destroy_on_stack() annotation into it.

The flag is horrible as well. How about this:

/* helper function, do not use in code ! */
__bdi_work_init(....., onstack)
{
	....
	if (on_stack) {
	   work.state |= WS_ONSTACK;
	   init_rcu_head_on_stack(&work.rcu_head);
	} else {
	....
}

See, how this moves also the "work.state |= WS_ONSTACK;" line out of
the calling code.

bdi_work_init(...)
{
	 __bdi_work_init(...., false);
}

bdi_work_init_on_stack(...)
{
	 __bdi_work_init(...., true);
}


out of the code.

To make it complete, please do not use the asymmetric:

   destroy_rcu_head_on_stack(&work.rcu_head);

Create a helper function:

bdi_destroy_work_on_stack(...)
{
   destroy_rcu_head_on_stack(work->rcu_head);
}    

That makes it way more readable and we did that with the other on
stack initializers as well.

Thanks,

	tglx