linux-kernel - Re: [PATCH 08/14] tty/serial/kgdboc: Add and wire up clear

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Date:	Mon, 10 Sep 2012 10:57:04 -0700
From:	Anton Vorontsov <anton.vorontsov@...aro.org>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Russell King <linux@....linux.org.uk>,
	Jason Wessel <jason.wessel@...driver.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Alan Cox <alan@...ux.intel.com>,
	Arve Hjønnevåg <arve@...roid.com>,
	Colin Cross <ccross@...roid.com>,
	Brian Swetland <swetland@...gle.com>,
	John Stultz <john.stultz@...aro.org>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linaro-kernel@...ts.linaro.org, patches@...aro.org,
	kernel-team@...roid.com, kgdb-bugreport@...ts.sourceforge.net,
	linux-serial@...r.kernel.org
Subject: Re: [PATCH 08/14] tty/serial/kgdboc: Add and wire up clear_irqs
 callback

On Mon, Sep 10, 2012 at 12:16:24PM +0100, Alan Cox wrote:
> > serial port, the CPU receives NMI exception, and we fall into KDB
> > shell.  So, it is our "debug console", and it is able to interrupt
> > (and thus debug) even IRQ handlers themselves.
> 
> You seem to have an assumption of single core here. What happens if
> the NMI hits CPU #0 and the serial IRQ hits CPU #1 simultaneously ?

If you can't redirect all serial IRQs to NMI context, e.g. sometimes you get
NMIs, sometimes IRQs, then your NMI handling is not deterministic, and surely
this is not supported.

The whole concept of clearing IRQs is needed if serial IRQ is routed to the
NMI/FIQ (on all CPUs), which by definition guarantees that serial IRQ routine
is never triggered. We "steal" all and every serial port's IRQs.

Thanks!
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