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SubjectRe: Question about in_interrupt() semantics with regard to softirqs.
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On Thu, Dec 30, 2010 at 4:27 AM, Justin Seyster <jrseys@gmail.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

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