netdev - Re: Latest PreemptRT patch error on imx35

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Date:	Mon, 31 Aug 2009 15:22:00 +0200
From:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
To:	Tim Sander <tim01@...i.informatik.tu-darmstadt.de>
Cc:	linux-rt-users <linux-rt-users@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Greg Ungerer <gerg@...inux.org>, netdev@...r.kernel.org
Subject: Re: Latest PreemptRT patch error on imx35

Hello Tim,

> I just bootet 2.6.31-rc8-rt9 on the phycore-imx35 development board, 
> without any additional patches. The FEC Ethernet Controller went belly 
> up, but the system booted nevertheless. [...]
The problem is that &fep->mii_lock is taken twice.  On a normal (i.e.
non-RT) UP system that doesn't hurt because spinlocks are noops (unless
you have some debugging code activated).

The exact problem is:
	fec_enet_mii (this takes &fep->mii_lock) and calls mip->mii_func
	which happens to be mii_queue.  mii_queue in turn takes
	&fep->mii_lock again.

The correct fix is probably to split mii_queue into a function that
locks and a function that does the actual work.  And then use the latter
in fec_enet_mii.

I'd prepare a patch but my wife just called me to eat some cake :-)

So maybe later today or tomorrow, we'll see.

Best regards
Uwe

-- 
Pengutronix e.K.                              | Uwe Kleine-König            |
Industrial Linux Solutions                    | http://www.pengutronix.de/  |
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