linux-kernel - Re: [rtc-linux] Re: [PATCH] rtc: Make rtc_update

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Message-Id: <200904231245.33435.david-b@pacbell.net>
Date:	Thu, 23 Apr 2009 12:45:32 -0700
From:	David Brownell <david-b@...bell.net>
To:	Alessandro Zummo <alessandro.zummo@...ertech.it>
Cc:	rtc-linux@...glegroups.com, Atsushi Nemoto <anemo@....ocn.ne.jp>,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	hcegtvedt@...el.com, vapier@...too.org, rongkai.zhan@...driver.com,
	balajirrao@...nmoko.org, broonie@...nsource.wolfsonmicro.com
Subject: Re: [rtc-linux] Re: [PATCH] rtc: Make rtc_update_irq callable with irqs enabled

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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