linux-kernel - Re: [patch 2.6.28-rc2] at91

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Date:	Tue, 28 Oct 2008 18:36:56 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	David Brownell <david-b@...bell.net>
Cc:	Pierre Ossman <drzeus-mmc@...eus.cx>,
	lkml <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Russell King <rmk@....linux.org.uk>,
	Nicolas Ferre <nicolas.ferre@....atmel.com>,
	Andrew Victor <linux@...im.org.za>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch 2.6.28-rc2] at91_mci: workaround lockdep

On Tue, 2008-10-28 at 10:22 -0700, David Brownell wrote:
> On Tuesday 28 October 2008, Peter Zijlstra wrote:
> > On Mon, 2008-10-27 at 14:26 -0700, David Brownell wrote:
> > > From: David Brownell <dbrownell@...rs.sourceforge.net>
> > > 
> > > Lockdep reported a problem in the at91_mci driver ... in this case, the
> > > issue is with lockdep, not with the driver. ...
> > > 
> > > When __flush_dcache_aliases() returns -- inlined into flush_dcache_page(),
> > > above -- it re-enables IRQs ... since that evidently may only be called with
> > > IRQs enabled.  That's OK since the (unshared) IRQ handler doesn't ask for IRQs
> > > to be disabled.   Except ... that lockdep went and disabled them, then went on
> > > to complains about the breakage *it* caused!
> > > 
> > > Workaround: depend on LOCKDEP=n ... 
> > 
> > In all previous such cases it was deemed the IRQ handler should deal
> > with whatever it gets.
> 
> In which case I'll wait until someone changes that IRQ handler (or that
> ARM MM utility, or lockdep), and give up using AT91 platforms for sanity
> testing kernel changes; lockdep is important, when it doesn't lie.
> 
> I do think that lockdep should warn when that it's ignoring such driver
> requests, however.  I seem to have been tripping over it a lot lately,
> and knowing that IRQ handlers were using strange modes would have saved
> a bunch of time from being wasted.
> 
> Threaded IRQ handlers are going to need to rely even more on running
> with IRQs enabled ... not to mention needing to sleep.  So it's clear
> to me that there *are* lockdep issues yet to be adressed here.

Sure, care so send a patch fixing those? :-)

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