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    SubjectRe: current_thread_info() vs task_thread_info(current)
    On Tue, Jul 19, 2011 at 07:36:23AM +1000, Benjamin Herrenschmidt wrote:
    > On Mon, 2011-07-18 at 07:37 -0700, Paul E. McKenney wrote:
    > > 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.
    >
    > 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.

    Thanx, Paul


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