Subject: Re: [RFC][PATCH] irq: remove IRQF_DISABLED
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@elte.hu>,
       Thomas Gleixner <tglx@linutronix.de>,
       lkml <linux-kernel@vger.kernel.org>,
       linux-arch <linux-arch@vger.kernel.org>,
       Andrew Morton <akpm@linux-foundation.org>
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Date: Fri, 06 Mar 2009 19:58:42 +1100
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On Mon, 2009-03-02 at 09:11 -0800, Linus Torvalds wrote:
> 
> On Mon, 2 Mar 2009, Peter Zijlstra wrote:
> > 
> > Would you be willing to take such a patch?
> 
> Yes - some day.
> 
> The "irq's disabled fastpath" thing has been there since pretty much day 
> one, because some irq handlers always wanted it. Making it the default 
> (and the only choice) is fine.

 .../...

I tend to disagree... (not -that- strongly but I felt like saying it
anyway :-) some archs have a reasonably nice support in the PIC for
interrupt priorities, allowing higher priority interrupts to "preempt"
lower priority ones, which this would effectively render useless.

Also, while yes, I agree, interrupts handlers -should- be short in
practice IDE is far from being the only example where this is not the
case and so we would delay timer interrupts for example for a
significant amount of time (or serial, that's another good example).

Also, we use the priority on some platform to have a high priority used
as a kind of "debugger" NMI .. ie, we don't have a real NMI but it's
better than nothing and here too, this would break it.

I don't see us having such a strong benefit from this... in fact, with
things like -rt, interrupts get moved to threads no ? Thus they
typically run with interrupts enabled... why have a different behaviour
on non-rt ? or I am missing something ? (I'm not terribly familiar with
the -rt stuff here so I probably am missing something).

Cheers,
Ben.


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