linux-kernel - Re: Debugging Thinkpad T430s occasional suspend failure.

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Date:	Sun, 17 Feb 2013 21:58:16 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Dave Jones <davej@...hat.com>, Hugh Dickins <hughd@...gle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Paul McKenney <paul.mckenney@...aro.org>
Subject: Re: Debugging Thinkpad T430s occasional suspend failure.

2013/2/17 Linus Torvalds <torvalds@...ux-foundation.org>:
> On Sun, Feb 17, 2013 at 7:11 AM, Frederic Weisbecker <fweisbec@...il.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.

Download attachment "patch2.diff" of type "application/octet-stream" (3107 bytes)