linux-kernel - Re: [patch 0/2] Run interrupt handlers always with interrupts disabled

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Message-ID: <20100326105358.GA4424@elte.hu>
Date:	Fri, 26 Mar 2010 11:53:58 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Nicolas Pitre <nico@...xnic.net>,
	Linus Torvalds <torvalds@...l.org>,
	LKML <linux-kernel@...r.kernel.org>, linux-arch@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	David Miller <davem@...emloft.net>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Russell King <rmk@....linux.org.uk>
Subject: Re: [patch 0/2] Run interrupt handlers always with interrupts
 disabled


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Fri, 2010-03-26 at 09:59 +0000, Alan Cox wrote:
> > > As long as it's rare (which it is) i dont see a problem: you can enable 
> > > interrupts in the handler by using local_irq_enable(), like the IDE PIO 
> > > drivers do. That way it's documented a bit better as well, because it shows 
> > > the precise source of the latency, with a big comment explaining it, etc.
> > 
> > I don't think it's as rare as you think particularly in embedded, and the 
> > moment you start explicitly using local_irq_enable() you've simply moved 
> > the underlying problem back and made it far harder to grep for.
> 
> We've got local_irq_enable_in_hardirq() which should be used and can easily 
> be grep'ed for.
> 
> But yes, I would much prefer to simply convert these known slow handlers to 
> threaded interrupts.

Yeah, agreed. So there's multiple solutions:

 - On old hw with a driver that nobody is willing to convert to threaded IRQs: 
   use the existing local_irq_enable_in_hardirq() API. This preserves the 
   status quo.

 - On new hw with new drivers where there's such a level of IRQ parallelism 
   that enabling IRQs in hardirqs is not an option, use threaded IRQ handlers.

Thanks,

	Ingo
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