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SubjectRe: Debugging Thinkpad T430s occasional suspend failure.
From
2013/2/17 Linus Torvalds <torvalds@linux-foundation.org>:
> On Sun, Feb 17, 2013 at 7:11 AM, Frederic Weisbecker <fweisbec@gmail.com> wrote:
>>
>> preempt_value_in_interrupt() looks buggy in your patch: it makes
>> invoke_softirq() returning if (val & HARDIRQ_MASK). But that's always
>> true since you have moved further the sub_preempt_count(IRQ_EXIT)
>> further.
>
> No, that's not it. The value hasn't been written back yet, but it already did:
>
> + int offset = IRQ_EXIT_OFFSET;
> + int count = preempt_count() - offset;
>
> so the 'count' has the IRQ_EXIT_OFFSET already subtracted. So no,
> HARDIRQ_MASK is *not* always set.

Another thing. Perhaps we can push the idea of your patch a little
further by re-entering HARDIRQ_OFFSET at the end of the softirq
processing and do the final sub_preempt_count(HARDIRQ_OFFSET) at the
very end of irq_exit().

This way irq_exit() looks a bit more simple to me: everything there
becomes considered as in hardirq: (ie: rcu_irq_exit() and
tick_nohz_irq_exit() won't appear anymore as weird special cases) and
we get rid of that IRQ_EXIT_OFFSET hack that fixes the CONFIG_PREEMPT
case.

I'm attaching an untested patch that modify yours. It's probably
missing some corner cases that rely on in_interrupt() value in
rcu_irq_exit() and tick_nohz_irq_exit() and may be other things.
[unhandled content-type:application/octet-stream]
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