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

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Message-ID: <48AD7705.2000404@s5r6.in-berlin.de>
Date:	Thu, 21 Aug 2008 16:09:09 +0200
From:	Stefan Richter <stefanr@...6.in-berlin.de>
To:	Nick Piggin <nickpiggin@...oo.com.au>
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

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.

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.)
-- 
Stefan Richter
-=====-==--- =--- =-=-=
http://arcgraph.de/sr/
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