linux-kernel - Re: [PATCH 1/2] genirq: Introduce a new flag IRQ_IS

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Message-ID: <alpine.DEB.2.11.1510022212000.4500@nanos>
Date:	Fri, 2 Oct 2015 22:16:49 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Mika Westerberg <mika.westerberg@...ux.intel.com>
cc:	Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>,
	Jiang Liu <jiang.liu@...ux.intel.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] genirq: Introduce a new flag IRQ_IS_CHAINED for
 chained interrupts

On Fri, 2 Oct 2015, Mika Westerberg wrote:

> In some cases it is useful to know if the interrupt in question has chained
> handler installed. For example when a cpu is offlined the architecture code
> needs to know if it has any users so that it can fixup affinity
> accordingly.
> 
> To make this possible we introduce a new flag IRQ_IS_CHAINED that is set by
> the core code when chained interrupt handler is installed. We also make it
> possible for core and architecture code to check the flag by introducing
> function irq_has_chained_handler() for this.

While looking at that patch it occured to me, that we can solve this
in a different way, which does not require any changes to the
migration code.

When we install the chained handler, we set the action of that irq
descriptor to a statically allocated chained_action which provides a
handler which emits a fat warning. chained handlers should never end
up calling an action and if they do, it's clearly a bug.

The availability of an action makes the migration code just pick it
and move it along. And that fixes all architectures in one go without
touching them.

Sorry, that I didn't think about this right away.

Thanks,

	tglx

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