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

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Date:	Tue, 3 Mar 2009 01:44:27 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	David Brownell <david-b@...bell.net>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	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, tglx@...utronix.de
Subject: Re: lockdep and threaded IRQs (was: ...)


* David Brownell <david-b@...bell.net> wrote:

> On Monday 02 March 2009, Ingo Molnar wrote:
> > > 
> > > The significant omission is lack of support for chaining
> > > such threads.  Example, an I2C device that exposes
> > > several dozen IRQs with mask/ack/... operations that
> > > require I2C access.
> > 
> > Well, those are rarely used, embedded-only constructs - the main 
> > focus of IRQ threading patches are the more common patterns.
> 
> Yes, mostly for embedded, where "system bus" more likely
> means I2C than PCI.
> 
> 
> > Since you care about them - could you please send patches on top 
> > of the IRQ threading patches to add support for them?
> 
> I'll look at that, and try to prepare something on top
> of the version of the threading patches that gets into
> the -next tree.  I got the impression there was going
> to be a v3 of those patches soonish...

Great! We'll sort out any conflicts so dont worry about that - 
you can pick up v2 just fine and post patches.

> I expect there will be two basic parts of that work:
> 
>  - One to cope with the upcoming change to handle_irq(),
>    insisting that it live in hardirq context instead of
>    just an irqs-off context (and thereby preventing use
>    of standard chaining calls in irq threads, sigh).
> 
>  - Another to set up a chaining thread, since chain
>    setup bypasses setup_irq() and friends.

If you mean to push the chaining bits into the IRQ thread too, i 
think the chaining bits actually should never be threaded. Is 
there a good reason to do that? It's not like they will really 
be preemptible (preempting a chaining thread would mean the 
whole demuxing chain is held up => bad).

> That latter might touch what the v2 patches added,
> since I'd want it to share code.

Sure.

> 
> - Dave
> 
> p.s. Note that those changes would still leave the
>      lockdep bug around ... it will still be breaking
>      various drivers that use normal IRQs, by forcibly
>      enabling IRQF_DISABLED.

it's not a bug - and i think Peter explained that already. It's 
not really breaking things either - we've had this for more than 
2 years.

	Ingo
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