linux-kernel - Re: current_thread_info() vs task_thread

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Message-ID: <20110718143731.GA2312@linux.vnet.ibm.com>
Date:	Mon, 18 Jul 2011 07:37:31 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Peter Zijlstra <peterz@...radead.org>, linux-arch@...r.kernel.org,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: current_thread_info() vs task_thread_info(current)

On Mon, Jul 18, 2011 at 09:54:57PM +1000, Benjamin Herrenschmidt wrote:
> On Mon, 2011-07-18 at 13:23 +0200, Peter Zijlstra wrote:
> 
> > So how are we going to solve this? Naively I'd think that
> > current_thread_info() is short for task_thread_info(current), and thus
> > the platforms for where this isn't true are broken.
> > 
> > I mean, what use is the thread_info not of a thread?
> > 
> > Comments?
> 
> Thomas just hit a bug in the platform code of said platform (powerpc
> heh ?) :-)
> 
> We do it right for hard IRQs and for some reason never did it right for
> softirqs.
> 
> The code is like this for the former:
> 
> static inline void handle_one_irq(unsigned int irq)
> {
> 
>         .../...
> 
> 	call_handle_irq(irq, desc, irqtp, desc->handle_irq);
> 	current->thread.ksp_limit = saved_sp_limit;
> 	irqtp->task = NULL;
> 
> 	/* Set any flag that may have been set on the
> 	 * alternate stack
> 	 */
> 	if (irqtp->flags)
> 		set_bits(irqtp->flags, &curtp->flags);
> }
> 
> So what we need, I suppose is to add those two last line to
> do_softirq_onstack() as well.

Hmmm...  Would this explain preempt_count() inexplicably increasing by
three across a spin_unlock_irqrestore()?  I ran into this situation when
testing on Power over the weekend.

							Thanx, Paul

> Now indeed i386 needs a similar treatment on both hard and soft
> irqs (along with getting rid of that stupid duplication of
> call_on_stack in there, I don't think it's worth making the code
> horrible like that to save one clobber and PeterZ reckons we can
> probably avoid it using always_inline anyways).
> 
> I'll let you guys sort i386 out tho, I'll look at fixing ppc tomorrow :-)
> 
> Cheers,
> Ben.
> 
> 
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