linux-kernel - Get rid of IRQF_DISABLED - (was [PATCH] genirq: warn about IRQF_SHARED|IRQF

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Date:	Mon, 30 Nov 2009 14:54:54 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	David Brownell <dbrownell@...rs.sourceforge.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>, Nicolas Pitre <nico@...vell.com>,
	Eric Miao <eric.y.miao@...il.com>,
	John Stultz <johnstul@...ibm.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Remy Bohmer <linux@...mer.net>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Andrea Gallo <andrea.gallo@...ricsson.com>,
	Jamie Lokier <jamie@...reable.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Get rid of IRQF_DISABLED - (was [PATCH] genirq: warn about
 IRQF_SHARED|IRQF_DISABLED)

On Mon, 30 Nov 2009, Uwe Kleine-König wrote:
> For shared irqs IRQF_DISABLED is only guaranteed for the first handler.
> So only warn starting at the second registration.
> 
> The warning is moved to __setup_irq having the additional benefit of
> catching actions registered using setup_irq not only register_irq.
> 
> This doesn't fix the cases where setup order is wrong but it should
> report the broken cases more reliably.

The whole IRQF_DISABLED trickery is questionable and I'm pretty
unhappy about the warning in general.

While it is true that there is no guarantee of IRQF_DISABLED on shared
interrupts (at least not for the secondary handlers) we really need to
think about the reason why we want to run interrupt handlers with
interrupts enabled at all.

The separation of interrupt handlers which run with interrupts
disabled/enabled goes all the way back to Linux 1.0, which had two
interrupt handling modes:

1) handlers installed with SA_INTERRUPT ran atomically with interrupts
   disabled.

2) handlers installed without SA_INTERRUPT ran with interrupts enabled
   as they did more complex stuff like signal handling in the kernel.

The interrupt which was always run with interrupts disabled was the
timer interrupt because some of the "slower" interrupt handlers were
relying on jiffies being updated, which is only possible when they run
with interrupts enabled and no such handler can interrupt the timer
interrupt.

In the 2.1.x timeframe the discussion about shared interrupt handlers
and the treatment of SA_INTERRUPT (today IRQF_DISABLED) was resolved
by changing the code to what we have right now. If you read back in
the archives you will find the same arguments as we have seen in this
thread and a boatload of different solutions to that.

The real question is why we want to run an interrupt handler with
interrupts enabled at all. There are two reaons AFAICT:

1) interrupt handler relies on jiffies being updated:

   I don't think that this is the case anymore and if we still have
   code which does it is probably historic crap which is unused for
   quite a time.

2) interrupt handler runs a long time:

   I'm sure we still have some of those especially in the
   archaelogical corners of drivers/* and in the creative space of the
   embedded "oh, I don't know why but it works" departement. That's
   code which needs to be fixed anyway.

The correct solution IMNSHO is to get rid of IRQF_DISABLED and run
interrupt handlers always with interrupts disabled and require them
not to reenable interrupts themself.

Thoughts ?

	 tglx