linux-kernel - Re: lockdep and threaded IRQs (was: ...)

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Date:	Fri, 27 Feb 2009 21:12:03 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	David Brownell <david-b@...bell.net>
Cc:	me@...ipebalbi.com, linux-kernel@...r.kernel.org,
	linux-input@...r.kernel.org, felipe.balbi@...ia.com,
	dmitry.torokhov@...il.com, sameo@...nedhand.com,
	a.p.zijlstra@...llo.nl, tglx@...utronix.de
Subject: Re: lockdep and threaded IRQs (was: ...)

On Fri, 27 Feb 2009 20:46:50 -0800 David Brownell <david-b@...bell.net> wrote:

>   drivers/mfd/twl4030-irq.c
> 
> Where you'll observe twl_init_irq() at line 688 setting
> up the thread and the Primary IRQ Handler (PIH) dispatch.
> That's pretty much bog-standard chained IRQ setup code,
> except that it chains through a thread.

OK, that's clever.  I never knew that anyone was doing that.  afaict
MFD is the only such place...

Yes, it's regrettable that it's a private-to-mfd implementation.  I
expect a lot of i2c clients (at least) would like this.

> When an IRQ comes in, handle_twl4030_pih() acks and masks
> that top level IRQ.  Then it wakes twl4030_irq_thread(),
> which issues I2C operations to read the IRQ status from
> the chip ... first PIH to find out which SIH modules are
> raising an IRQ, then SIH to dispatch that status.  Then
> handle_irq() from that thread to invoke the handler in
> that thread context; it will issue more I2C ops.

yup.

> And the lockdep thing kicks in through handle_irq(),
> where the IRQ handler wrongly gets invoked with the
> IRQs disabled -- iff lockdep is enabled.  Otherwise,
> that IRQ thread is just like any other thread.

OK.

Perhaps it would be somewhat less dirty to do something like

--- a/kernel/irq/manage.c~a
+++ a/kernel/irq/manage.c
@@ -689,7 +689,8 @@ int request_irq(unsigned int irq, irq_ha
 	/*
 	 * Lockdep wants atomic interrupt handlers:
 	 */
-	irqflags |= IRQF_DISABLED;
+	if (!(irqflags & IRQF_NO_LOCKDEP_HACK))
+		irqflags |= IRQF_DISABLED;
 #endif
 	/*
 	 * Sanity-check: shared interrupts must pass in a real dev-ID,
_

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