linux-kernel - 2.6.18-rc4 i386 CONFIG_4KSTACKS does not use the hardirq stack

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Message-ID: <7109.1156475797@kao2.melbourne.sgi.com>
Date:	Fri, 25 Aug 2006 13:16:37 +1000
From:	Keith Owens <kaos@....com.au>
To:	linux-kernel@...r.kernel.org
Cc:	"Ingo Molnar" <mingo@...e.hu>,
	"Thomas Gleixner" <tglx@...utronix.de>
Subject: 2.6.18-rc4 i386 CONFIG_4KSTACKS does not use the hardirq stack

2.6.18-rc4 arch/i386/kernel/irq.c::do_IRQ has this code:

	if (!irq_desc[irq].handle_irq) {
		__do_IRQ(irq, regs);
		goto out_exit;
	}
#ifdef CONFIG_4KSTACKS

	curctx = (union irq_ctx *) current_thread_info();
	irqctx = hardirq_ctx[smp_processor_id()];
	...

All registered irqs on my system have a NULL handle_irq so do_IRQ
always calls __do_IRQ on the current stack, it never switches to the
hardirq_ctx stack.  Removing the test is trivial and it boots and runs
without the test for !irq_desc[irq].handle_irq, but why was the test
put there in the first place?

Note: with that test removed, do_IRQ runs on the hardirq stack, but
struct thread_info is not updated accordingly so backtrace is wrong.  I
can fix that as well, but I need to understand the test for handle_irq
first.

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