linux-kernel - cpu hotplug and lockdep (was: Re: [tip:core/ipi] generic-ipi: eliminate spurious pointlessWARN

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Message-Id: <1237577688.4667.68.camel@laptop>
Date:	Fri, 20 Mar 2009 20:34:48 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Jan Beulich <jbeulich@...ell.com>, tglx@...utronix.de,
	mingo@...hat.com, hpa@...or.com, linux-kernel@...r.kernel.org,
	Gautham R Shenoy <ego@...ibm.com>
Subject: cpu hotplug and lockdep (was: Re: [tip:core/ipi] generic-ipi:
 eliminate spurious pointlessWARN_ON()s)

On Fri, 2009-03-20 at 19:52 +0100, Peter Zijlstra wrote:
> On Fri, 2009-03-20 at 19:24 +0100, Ingo Molnar wrote:
> > * Eric W. Biederman <ebiederm@...ssion.com> wrote:
> > 
> > > Now back to my regularly scheduled hotunplug challenges starting 
> > > with getting lockdep to warn about the dead locks.
> 
> http://programming.kicks-ass.net/kernel-patches/cpu-hotplug/
> 
> The recursive read code needs more work.. 

You'll find some network softirq inversion 'fixes' there, but those are
false, (soft)irq recursion isn't a problem for recursive read locks, but
of course only as long as writes (and thereby all nesting exclusive
locks) are irq-safe so as to not create write in read deadlocks.

Just never gotten around to actually implementing that, ego also offered
to look at that at one point, but I guess he didn't get around to it
either ;-)

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