linux-kernel - Re: Question about in_interrupt() semantics with regard to softirqs.

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Message-ID: <AANLkTi=TR+gO6kdA1M5XkF-MTaUVBzXXtngmpNmwTYuX@mail.gmail.com>
Date:	Thu, 30 Dec 2010 10:15:52 +0800
From:	Yong Zhang <yong.zhang0@...il.com>
To:	Justin Seyster <jrseys@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Question about in_interrupt() semantics with regard to softirqs.

On Thu, Dec 30, 2010 at 4:27 AM, Justin Seyster <jrseys@...il.com> wrote:
> I'm trying to understand the in_interrupt() function, and it seems
> that it will return true for normal, non-interrupt code that disables
> bottom half processing.  It looks like that behavior is intentional,
> but I don't understand why it's designed that way.  I'm sorry if I'm
> stating something obvious here; it would help me a lot if somebody
> double checked my reasoning!
>
> in_interrupt() checks a hardirq count and a softirq count, but I found
> out that these two counts behave very differently.  The hardirq count
> tracks the nesting depth of hardware interrupts (which is what I would
> expect), but the softirq count behaves like the preempt count,
> tracking whether softirqs are currently enabled.
>
> So if normal code (executing on behalf of a user process) disables
> softirqs with local_bh_disable(), it will get a true return value from
> in_interrupt() until it finally reenables them.  But disabling
> hardirqs will not have the same effect: the hardirq count is
> unchanged, and in_interrupt() will still return false.
>
> My question is: is there a design decision for this asymmetry between
> hard and softirqs?  Also, is there a function that does what I really
> wanted, which is to return true iff execution is actually in
> bottom-half context?

For the function you want, you can take a look at commit:
75e1056f5c57050415b64cb761a3acc35d91f013

Thanks,
Yong

-- 
Only stand for myself
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