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Date: Sun, 17 Feb 2013 22:02:11 +0100
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Subject: Re: Debugging Thinkpad T430s occasional suspend failure.
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>, Dave Jones <davej@redhat.com>,
        Hugh Dickins <hughd@google.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Paul McKenney <paul.mckenney@linaro.org>
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2013/2/17 Frederic Weisbecker <fweisbec@gmail.com>:
> 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.

I messed up preempt_offset_in_interrupt() with in_atomic() code
instead of in_interrupt(). Anyway the patch is untested and is more
there to get your opinion for now. I'll put some more care on it if
people like it.