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

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Message-ID: <20080821114745.GD21089@linux.vnet.ibm.com>
Date:	Thu, 21 Aug 2008 04:47:45 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Stefan Richter <stefanr@...6.in-berlin.de>,
	jmerkey@...fmountaingroup.com, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	David Howells <dhowells@...hat.com>
Subject: Re: [ANNOUNCE] mdb: Merkey's Linux Kernel Debugger 2.6.27-rc4
	released

On Thu, Aug 21, 2008 at 01:02:48PM +0200, Peter Zijlstra wrote:
> On Thu, 2008-08-21 at 12:57 +0200, Stefan Richter wrote:
> > Peter Zijlstra wrote:
> > > On Wed, 2008-08-20 at 20:50 -0600, jmerkey@...fmountaingroup.com wrote:
> > > 
> > >>  volatiles left in the code due to the previously stated
> > >>  (and still present) severe breakage of the GNU compiler with SMP 
> > >>  shared data.  most of the barrier() functions are just plain broken
> > >>  and do not result in proper compiler behavior in this tree. 
> > > 
> > > Can you provide explicit detail?
> > > 
> > > By using barrier() the compiler should clobber all its memory and
> > > registers therefore forcing a write/reload of the variable.
> > 
> > I hope Jeff didn't try mere barrier()s only.  smp_wmb() and smp_rmb()
> > are the more relevant barrier variants for mdb, from what I remember
> > when I last looked at it.
> 
> Sure, but volatile isn't a replacement for memory barriers.

Let's face it, the C standard does not support concurrency, so we are
all in a state of sin in any case, forced to rely on combinations of
gcc-specific non-standard language extensions and assembly language.

Could be worse!!!

							Thanx, Paul
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