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

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Message-Id: <20090425.014850.93020801.anemo@mba.ocn.ne.jp>
Date:	Sat, 25 Apr 2009 01:48:50 +0900 (JST)
From:	Atsushi Nemoto <anemo@....ocn.ne.jp>
To:	alessandro.zummo@...ertech.it
Cc:	rtc-linux@...glegroups.com, david-b@...bell.net,
	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 Fri, 24 Apr 2009 13:13:34 +0200, Alessandro Zummo <alessandro.zummo@...ertech.it> wrote:
> > >  use spin_lock() in the IRQ handler and spin_lock_irq/irq_save
> > >  in the setup functions.  
> > 
> > I think you're describing how the *current* scheme is supposed
> > to work ... except that some IRQ handlers aren't calling the
> > rtc_update_irq() routine with IRQs blocked.
> > 
> > Yes, that current scheme works ... modulo those buggy handlers.
> 
>  ok, but why it's necessary to disable the interrupts? Only because
>  the specs says so or because there's a locking issue I'm missing?

Here is a possible example:

1. RTC alarm interrupt handler takes rtc->irq_lock by spin_lock()
2. A timer interrupt handler calls rtc_uie_timer() for UIE emulation
3. rtc_uie_timer() waits on rtc->irq_lock .... deadlock!

This sort of deadlock can happen if a spin lock was used by multple
interrupt handlers.

---
Atsushi Nemoto
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