linux-kernel - Re: current_thread_info() vs task_thread

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Message-ID: <20110719030404.GA2355@linux.vnet.ibm.com>
Date:	Mon, 18 Jul 2011 20:04:04 -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 Tue, Jul 19, 2011 at 10:57:37AM +1000, Benjamin Herrenschmidt wrote:
> On Mon, 2011-07-18 at 17:39 -0700, Paul E. McKenney wrote:
> > > Hrm, no I don't see that happening no. The preempt count when
> > exiting an
> > > irq or softirq stack should be the exact same as when entering it,
> > which
> > > is why we don't bother copying it over. Do you see any case where
> > that
> > > wouldn't hold ?
> > 
> > Nope, other than seeing preempt_count() transition from zero to three
> > across a spin_unlock_irqrestore() for no good reason that I could see.
> 
> Do you have a nice repro-case ? :-)
> 
> That sounds really nasty ... smells really like something bad's
> happening from an interrupt, but we don't copy back the preempt-count
> from the interrupt stacks at all, so that's really really odd.

Good question...  The system I reproduced on four times over the
weekend is out of commission.  Trying the same test on another system
with a minimal patch -- will let you know how it goes.

							Thanx, Paul
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