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From: David Brownell <david-b@pacbell.net>
To: Alessandro Zummo <alessandro.zummo@towertech.it>
Subject: Re: [rtc-linux] Re: [PATCH] rtc: Make rtc_update_irq callable with irqs enabled
Date: Thu, 23 Apr 2009 12:45:32 -0700
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Cc: rtc-linux@googlegroups.com, Atsushi Nemoto <anemo@mba.ocn.ne.jp>,
       akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
       hcegtvedt@atmel.com, vapier@gentoo.org, rongkai.zhan@windriver.com,
       balajirrao@openmoko.org, broonie@opensource.wolfsonmicro.com
References: <20090410005820.4fcfcc1f@i1501.lan.towertech.it> <200904231225.01618.david-b@pacbell.net> <20090423212639.7ba7cb0b@i1501.lan.towertech.it>
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On Thursday 23 April 2009, Alessandro Zummo wrote:
> 
> > If you're talking about a different patch, please forward...
> 
> ?no patch, just theory.
> 
> ?the question is, do we need IRQs disabled when
> ?calling rtc_update_irq?

If the spinlock is *ever* acquired with IRQs disabled,
it must *always* be acquired that way.

The typical use is ... from IRQ context, which will in
some cases mean IRQs disabled.  QED.


> ?and if yes, why? to prevent what?

Consider:  one context grabs spinlock with IRQs enabled.
IRQ arrives.  That context tries to grab that same lock,
from the same CPU.  ==> Self-deadlock.

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