linux-kernel - Re: [RFC] Fix 2.6.33 x86 regression to kgdb hw breakpoints

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Date:	Wed, 30 Dec 2009 17:39:08 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Jason Wessel <jason.wessel@...driver.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Alan Stern <stern@...land.harvard.edu>,
	"K.Prasad" <prasad@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC] Fix 2.6.33 x86 regression to kgdb hw breakpoints - due
	to perf API changes

On Sat, Dec 12, 2009 at 03:01:58PM -0600, Jason Wessel wrote:
> Ingo Molnar wrote:
> > Basically we have two options:
> >
> >  A- change kgdb to use the hw-breakpoints highlevel APIs (i'd prefer 
> >     that)
> >
> >   
> 
> Right now we can't because the high level code uses all sorts of mutexes
> and sync points to get the hw breakpoints installs on the various
> processors.  After I re-spun my RFC patch, I found another problem.  I
> do use the high level code to create a block of 4 (struct perf_event **)
> structures, but doing so ultimately calls the reserve hw breakpoint even
> though they are marked as disabled when created.
> 
> Should I, or can I change that behavior?



We could probably have a helper that allocates a disabled breakpoint
without reserving it. But the problem remains: you'll need to take
locks when you eventually reserve it and when you activate it.

The fact that it can happen from nmi is really a problem.

Is there any possibility that we know the user has started a
kgdb session, and then reserve as much hardware breakpoints
as we can in kgdb at this time?

Thanks.

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