From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Mike Frysinger" <vapier.adi@gmail.com>
Subject: Re: local_save_flags(flags)
Date: Thu, 18 Sep 2008 18:51:23 -0400
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	 <545834.68406.qm@web63405.mail.re1.yahoo.com>
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To: fundu_1999@yahoo.com
Cc: linux embedded <linux-embedded@vger.kernel.org>

On Thu, Sep 18, 2008 at 18:45, Fundu wrote:
>> that book explicitly covers your question.  read chapter 2
>> where it
>> covers these irq functions.
>
> as i said i'm reading the book and actully i did read that chapter.
>
> for me, here's the exact line that needs clarification,
> In chapter 2, page 42 last paragraph(starts with "However, if ..." )
>
> here's the code snippet he's talking about.
> Point A:
> local_irq_disable();
> /* critical section ...*/
> local_irq_enable();
>
>
> Author say, if irg are already disabled at Point A (see snippet above) then local_irq_enable() creates an unpleasant side effect of re-enabling interrupts rather than restoring interrupt state.

this was the section i referred to ... but i guess it made perfect
sense to me since i'm familiar with the details

> 1) first what's the difference between re-enabling and restoring interrupt state.

many irq controllers have a global enable bit.  you can restore the
interrupt mask state without toggling that bit.

> 2) so is disable interrupts twice a problem, or just enabling them when after they are diabled (which sounds like how it should be ) a problem.

both are a problem.  the non-state saving version cannot be used
recursively nor in parallel to the state-saving version.
-mike