Re: [ANNOUNCE] mdb: Merkey's Linux Kernel Debugger 2.6.27-rc4released

[Peter Zijlstra]

On Thu, 2008-08-21 at 04:47 -0700, Paul E. McKenney wrote:
> 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@xxxxxxxxxxxxxxxxxxxxx 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.

Hehe, still, a little birdie told me they are working on it and perhaps
someone with clue could enlighten us on their direction.

Still, I'd like Jeff to show his C, the resulting asm and the intent for
the volatile and barrier versions of his code (well, little snippets of
his code obviuosly).

Either he doesn't understand barriers (nothing to be ashamed about), or
we might have more trouble lurking in the rest of the kernel.


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