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From: David Brownell <david-b@pacbell.net>
To: felipe.balbi@nokia.com
Subject: Re: [patch/rfc 1/2] GENIRQ: add handle_threaded_irq() flow handler
Date: Wed, 18 Mar 2009 11:31:22 -0700
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Cc: Ingo Molnar <mingo@elte.hu>,
       "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
       "tglx@linutronix.de" <tglx@linutronix.de>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>,
       "me@felipebalbi.com" <me@felipebalbi.com>,
       "dmitry.torokhov@gmail.com" <dmitry.torokhov@gmail.com>,
       "sameo@openedhand.com" <sameo@openedhand.com>
References: <1235762883-20870-1-git-send-email-me@felipebalbi.com> <200903171919.48030.david-b@pacbell.net> <20090318120056.GG24947@scadufax.research.nokia.com>
In-Reply-To: <20090318120056.GG24947@scadufax.research.nokia.com>
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On Wednesday 18 March 2009, Felipe Balbi wrote:
> > +?????action = desc->action;
> > +?????if (unlikely(!action || (desc->status & IRQ_DISABLED)))
> > +?????????????goto out_unlock;
> 
> you say below irqs are always enabled

Right here they're always disabled by spin_lock_irq().
The "below" follows spin_unlock_irq(), which re-enables
them to traverse that (locked) action list.


> so this branch is something we 
> never want to happen. How about adding a WARN() then ?

When some one says "irqs are enabled" they mean that,
local_irq_disable() or friends have not been called,
so for example a timer or other IRQ could arrive.

The IRQ_DISABLED flag in an IRQ descriptor means
something different:  "don't try *handling* this".

That particular check is used in *ALL* flow handlers.
It guards against things like races in disable_irq()
paths, which could allow an IRQ that was in flight
to arrive "after" the IRQ was disabled.

In the case of an IRQ enable/disable mask sitting
across an I2C bus boundary, it's particularly easy
to see how such a race might happen ... since both
the thread masking the IRQ, and the one handling it,
are subject to preemption and scheduling.
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