linux-kernel - Re: [PATCH] Delete redundant IRQ_DISABLED check in irq

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Date:	Sat, 18 Jul 2009 22:11:47 +0800
From:	Barry Song <21cnbao@...il.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	mingo@...hat.com, dahlmann.thomas@...or.de,
	LKML <linux-kernel@...r.kernel.org>,
	uclinux-dist-devel@...ckfin.uclinux.org,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] Delete redundant IRQ_DISABLED check in irq_thread

2009/7/18, Thomas Gleixner <tglx@...utronix.de>:
> On Sat, 18 Jul 2009, Barry Song wrote:
>> I guess people call disable_irq to disable the whole execution,
>> specially to disable the hardirq(then both top and bottom are
>> blocked). It's really strange the disable operation only blocks the
>> bottom-half.
>
> The disable blocks both. After the disable no hardirq comes in
> anymore.
>
I know disable_irq will block the future hardirq and bottom-half. Here
I means the current hardirq has been executed, but bottom-half will
not be executed because of the disable_irq. So the disable_irq only
blocks a half for the special case and the work of the current IRQ is
not finished fully in fact. But anyway, it should not cause trouble,
but just give the semantic of disable_irq a little confused.
Anyway, I agree it is not necessary to fulfill a complicated
check/sync for thread IRQ.

And thanks for your review and providing a good solution for my problem!

> The disabled check in the thread is just to take care of the case when
> the disable comes in between the hardirq and the thread handler.
>
> That way we avoid doing the full tracking of the hardirq / thread
> chain simply because it is complex and would produce significant
> overhead in the fast path for no gain. It could be done, but it's not
> worth the trouble.
>
> Thanks,
>
> 	tglx
>
>

Thanks,
Barry
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