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

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Date:	Sun, 1 Mar 2009 10:43:02 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	David Brownell <david-b@...bell.net>
cc:	Andrew Morton <akpm@...ux-foundation.org>, 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
Subject: Re: lockdep and threaded IRQs (was: ...)

On Sat, 28 Feb 2009, David Brownell wrote:
> That seems to presume a hardirq-to-taskirq handoff.  But the
> problem case is taskirq-to-taskirq chaining, through e.g.
> what set_irq_chip_and_handler() provided.  (Details not very
> amenable to brief emails, just UTSL.)
> 
> Thing is, I'm not sure a per-IRQ thread can work easily with
> that chaining.  The chained IRQs can need to be handled before
> the top-level IRQ gets re-enabled.  That's why the twl4030-irq
> code uses just one taskirq thread for all incoming events.

This can be solved by a completion as well.
 
> (Which of course is rarely more than one at a time, so there's
> little reason not to share that task between the demuxing code
> and the events being demuxed.  Interrupts that need processing
> via I2C/SPI/etc are more or less by definition not frequent or
> performance-critical.)

Then all we need to provide in the generic code is a function which
does not go through the handle_IRQ_event() logic and calls the action
handler directly. Not rocket science to do that and better than using
a facility which is designed to run in hardirq context and expect that
it works in thread context without complaints.

Thanks,

	tglx
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