linux-kernel - Re: [ANNOUNCE] mdb: Merkey's Linux Kernel Debugger 2.6.27-rc4 released

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Date:	Fri, 22 Aug 2008 11:40:24 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Stefan Richter <stefanr@...6.in-berlin.de>
Cc:	jmerkey@...fmountaingroup.com, paulmck@...ux.vnet.ibm.com,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>
Subject: Re: [ANNOUNCE] mdb: Merkey's Linux Kernel Debugger  2.6.27-rc4 released

On Friday 22 August 2008 00:09, Stefan Richter wrote:
> Nick Piggin wrote:
> > On Thursday 21 August 2008 22:26, jmerkey@...fmountaingroup.com wrote:
> >> It's simple to reproduce.  Take away the volatile declaration for the
> >> rlock_t structure in mdb-ia32.c (rlock_t debug_lock) in all code
> >> references and watch the thing lock up in SMP with multiple processors
> >> in the debugger each stuck with their own local copy of debug_lock.
> >
> > You should disable preempt before getting the processor id. Can't see any
> > other possible bugs, but you should be able to see from the disassembly
> > pretty easily.
>
> debug_lock() is AFAICS only called from contexts which have preemption
> disabled.  Last time around I recommended to Jeff to document this
> requirement on the calling context.

I'm not talking about where debug_lock gets called, I'm talking about
where the processor id is derived that eventually filters down to
debug_lock.

> But even though preemption is disabled, debug_lock() is still incorrect
> as I mentioned in my other post a minute ago.  It corrupts its .flags
> and .count members.  (Or maybe it coincidentally doesn't as long as
> volatile is around.)

I don't think so. And flags should only be restored by the processor
that saved it because the spinlock should disable preemption, right?
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