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On Thu, Aug 21, 2008 at 02:03:26PM +0200, Peter Zijlstra wrote: > 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@wolfmountaingroup.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.> > Hehe, still, a little birdie told me they are working on it and perhaps > someone with clue could enlighten us on their direction. Well, I guess you guys will be the judge of that. Or one of the judges, at least. ;-) One advantage of the current c++0x approach is that it allows extremely weak memory barriers to be used in many cases that would require smp_rmb() in current Linux kernel. If you are crazy enough to want to see a sneak preview in standardese, try all 10MB of: http://open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2691.pdf Section 1.10 (physical page 25, logical page 11) describes the memory model. Sections 29 and 30 describe the operations (physical page 1155, logical page 1141). The C and C++ guys got together ahead of time and agreed to work together towards a compatible solution. And rcu_dereference() would be implemented in terms of memory_order_consume, for whatever that is worth. > 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. Sounds fair to me! Thanx, Paul | ||||||||||
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